Comparing Better-Auth to Clerk or Auth0 misses the point entirely.
People choose Better-Auth because they want to own their user auth and users table themselves. Auth can be complex, but it's such a key and important piece of your business that outsourcing it to a 3rd party should be much closer to a last resort than a first impulse. If that 3rd party ever shuts down, has downtime, or your account gets suspended for whatever reason, users won't even be able to login to your app. That is a HUGE risk that I am not sure you are accounting for.
I get what you're saying, but the same can be said about any 3rd party vendor.
Even if you host your app on AWS or CloudFlare you can face some major downtimes like recently. Sure it's less likely for AWS to go out of business than some mom-and-pop auth service. But if you stick with major, overcharging, players like Auth0 you should be good.
Sure, it cost a lot, but I recently had a conversation with a entrepreneur who makes a really, really good life out of an online shop and she said something between the lines of "I fking hate shopify, they steal my money, I need to get out of that platform!". Sure they take her money, but they provide the whole foundation for her business, and the cut is like 3%. Payment provider takes similar cut. You can say they are leeching of her business, but without them - and she made a concious choice to only sell her products on her online platform - her business doesn't exist.
I have the same outlook at auth providers. Sure you can do it yourself, same as hosting, monitoring, etc. But most people these days decide to outsource that chore to someone else for a change. Auth0 et al. have a hefty free tier to lock you in, and then they charge you premium, but I'm very happy to take that bet in a not-so-serious scenario, because at the time I have to pay premium I'd better be making some serious money I can share with them.
Again - same argument can be made about AWS. It might be cheaper to own the infra, but in the transitional stage I likely won't be afford to staff a competent devops to do all the chores. And sure, couple steps later I'm locked in for life, but hey, if they didn't helped me at first I wouldn't be able to get there.
People choose Better-Auth because they want to own their user auth and users table themselves. Auth can be complex, but it's such a key and important piece of your business that outsourcing it to a 3rd party should be much closer to a last resort than a first impulse. If that 3rd party ever shuts down, has downtime, or your account gets suspended for whatever reason, users won't even be able to login to your app. That is a HUGE risk that I am not sure you are accounting for.