why are my alternators failing

Poor quality rebuilt alternators will do that. Once a long time ago I went through about 8 Pep Boys alternators in a short period of time. I got sick of playing the warranty game and bought a good one and never had another problem.

you may have a parasitic drain , also don't make a new alternator charge a dead batt. plus the " bad out of the box ' parts you get from " auto zoo " or " nipa " bite the bullet and buy from the dealer , test equipment would help . If you're going to work on your own vehicles there's tools you will need