If everything goes well you can buy a lovely house in the suburbs, raise 2.5 kids, grow old together with your wife and live happily ever after.
If it goes wrong she takes your kids and half your assets immediately, followed by several years of child support and possibly alimony.
It's not like you have a coin flip chance at a happy life or an unhappy divorce. You can influence the odds here. In fact, you can really stack the deck in your favor, so to speak.
Sure, people lie and people change, but it's not like you have to get married after a certain amount of time. You can wait to see what the person is like at their worst and then decide. Yeah, it's a 2 way street and you have to work with their timetable too. But no one is forcing you to just jump into a marriage blindly.
The problem is that there are 2 single points of failure in a marriage, and you only control one of them. I can be the best husband I can possibly be, work hard, provide for my family, love my wife with everything I have, and otherwise do what I can to try to make her happy. However, if she choses not to reciprocate this, there's literally nothing I can do about it. (You can change the genders here as appropriate, it's just that I'm a heterosexual male.)
The other problem is that people can change. Sometimes people go out of their way to be the very best version of themselves until they are married and then they just stop trying as hard or stop pretending to be the person the other person wants them to be. Also, people change as they get older, they have different priorities, different interests, new hobbies, changes in physical appearance, changes in libido, etc. After 10 or 20 years of marriage, it is very unlikely that you and your spouse haven't changed in some very significant ways.
Yo can look at it as a downside or as an upside.