But a much easier argument against sports betting is that it ruins the sports. Players throw. They get good at subtly cheating. The gambling apparatus latches itself to the sport, to the teams and players, the umpires and judges, the sporting organizations. With this much money on the line, it's not a matter of if but when games are thrown, cheated -- the bigger the game, the bigger the incentive. It's even easier now because of the amount of side/parlay betting that is available. It exhausts the spirit of competition.
Sports gambling is diametrically opposed to sport itself.
There's a false dichotomy between prohibition and laissez-faire, which the US seems particularly prone to. You've seen similar issues with the decriminalisation of cannabis, where many states seem to have switched abruptly from criminalisation to a fully-fledged commercial market. There is a broad spectrum of other options in between those points that tend to be under-discussed.
You can ban gambling advertising, as Italy did in 2019. You can set limits on maximum stakes or impose regulations to make gambling products less attractive to new customers and less risky for problem gamblers. You can have a single state-controlled parimutuel operator. Gambling does cause harm - whether it's legal or not - but it is within the purview of legislators to create a gambling market in which harm reduction is the main priority.
One regulation would be banning gambling advertising, for the same reason why smoking ads are (I think?) banned. It is especially nefarious how companies lure in new customers with free bets, often with unscrupulous cash-out conditions, in order to get people hooked. It’s the equivalent of ads providing someone a coupon code to get several boxes of free cigarettes, at which point they get hooked.
Another change I’d like to see is the end of mobile gambling. I’ve never done it, but from watching friends do it, it was far too easy to deposit money, or borrow money on credit, and bet it frivolously. At least if such behavior is confined to a casino, there is some larger barrier to entry for people.
I do not know if this is true in other states, but certain states have the ability for an individual to self-institute a gambling ban at all facilities in the state. I’m not sure if this applies to gambling online. If not, then it should. And if other states don’t have it, then they would greatly benefit from it.
It also seems somewhat fair to me to tax the casinos and other companies profiting from gambling and using that money to fund services for people who become addicted. If you’re going to help create a problem, you should have to help clean it up.
That's not an iron-clad argument, as legal gambling can still have mob ties, and tacit permission of some illegal gambling might still permit some level of oversight. And of course, legal gambling doesn't ensure reasonable or effective oversight or regulation.
By establishing known, legal, and possibly even bettor-favourable facilities or systems, gaming becomes something which might have some level of oversight. The increase in online gambling does severely cut into this argument though.
Another challenge, in the U.S., comes in the form of reservation casinos which can operate independently of other state prohibitions on gambling, which means that total eradication is at the very least difficult.
But that is an argument which might be made in answer to your "why not just..." question.
(I'm generally not a fan of gambling in any of its various forms. I'm cognisant of its pervasiveness and some of the worse aspects of it.)
I strongly suspect that one element of legalisation is that it normalises the activity, which lowers all sorts of social and psychological barriers to participation.
Another is that it creates self-organised self-interest groups. This is actually a really great way to ensure the longevity of governmental programmes, with both positive and negative examples: welfare systems such as Social Security, Medicare, and the ACA in the US are all immensely popular with the elderly, a staunch voting block, to the extent that its general trend toward conservativism doesn't fully mute interest in social welfare. The military-industrial complex is another, and a recent discussion I'd heard of the Inflation Reduction Act highlighted the constituencies built in to support it even in deep-red southern US states.
In the case of legalisation of gambling, drugs, and sex work, what had previously been the purview of criminal gangs now becomes "ordinary business" (though the thought occurs that the distinction between the two may be less than is commonly understood). To the extent that established businesses prove to be highly effective at defending even the most indefensible of practices (tobacco, alcohol, asbestos, lead, plastics, fossil fuels) is well established, and the risks of that path should be strongly considered.
Another option is to decriminalise rather than legalise a practice, but focus on policing the most problematic elements of the practice. That might be the provider side (as with drugs and gambling) or the consumer side (as with sex work, targeting johns), or on going up-market and tightly limiting or prohibiting private aggregators (e.g., pimps, drug lords) rather than focusing on low-level actors (streetwalkers, individual workers, street crews within drug operations).
State-operated operations (gambling, lotteries, alcohol and tobacco sales, drug distribution *with integrated treatment), is another option, though it too isn't a surefire solution. My view is that lottery programmes in the US are out of control and a net negative, though in part that itself reflects the public-private partnership in the operation of many of these.
The solution to this problem would be mandatory sex worker licenses and mandatory yearly counseling that acts as an escape path for trafficking victims.
There are many women police officers in vice and many means with which to tackle sex trafficking, with or without the testimony of specific victims (bearing in mind that sex trafficking almost always involves many victims).
Yearly contact seems ... sparse... there's more sense to be had in mandatory weekly or fortnightly STI checkups, etc. which incorporates contact with trained medical professionals familiar with the ins and outs of te game.