You're not wrong (ignoring how easy it is to hack DNS), but at the same time it's hard enough to get people to buy their own domain name, nevermind understand the system well enough to add a TXT record.
It's a strategy that's fine to implement when your target audience is server admins. It's a terrible strategy when your target audience is everyday users who you hope own their own domain. Doubly so in a world where owning your own domain is so rare for individuals.