It isn't trivial (you can't do it yourself) nor cheap (79€ for Samsung phones).

That's at most 1/10th the cost of the average Samsung phone.

That's cheap. If you think that a safe first-party replacement battery will sell for less than the 79€ that the whole replacement effort takes, then you're fooling yourself.

I strongly suspect that there's also not good language for blocking against third-party batteries (and the phone manufacturers would have good reason to do so because it might result in overheating or worse with really bad third-party batteries).