The vast majority of people are not comfortable opening their computer and mucking around the guts. In the old days Apple made this simple for their users:
https://www.ifixit.com/Guide/MacBook+Unibody+Model+A1278+Bat...
https://www.ifixit.com/Guide/MacBook+Unibody+Model+A1278+Bat...
That's why Apple offers to do it for them.
Sure, completely user-replacable would be ideal but such a battery design surely comes with other trade-offs and compromises.