Frequently over simplified. For example, a lot of those states require you to go get your ballot notarized, which can be a bit of a pain or a little cost:

https://www.usvotefoundation.org/absentee-ballot-notary-witness-requirements

Some require you to include a copy of your photo id.

Voting in person is the safest bet to make sure your vote counts and not get disqualified because of some rule you failed to notice/follow.