It's because healer jobs are made for everyone, not just top players.
Yes it gets more boring the better you become, but there are plenty of people out there who aren't on that level and they should be allowed to play a healing job aswell.
And no, you can't relegate one specific job to being the newbie one because either it will become so busted in the hands of higher level players it's the only healer you'll see in ultimates/savages or it's too newbie friendly and underperforms on higher level and never see it at all.
I get it, higher lvl players think it's too boring and lets say they make it more complex , but they fail to ever think about the lower skilled players, which most mmo players are lets be real and how it would affect them.
Mmo's already in general have this stigma of "healer and tank job hard", it's why you always see comments or posts from newer players who are afraid to do them and just go dps instead, not realizing tanking is by far the easiest and most braindead job in ff14 with healer not far behind.
That's why healers are the way they are in this game as are tanks, and honestly queue times show that it works, I feel out of all mmo's I played so far, queueing as dps is prob the fastest I ever seen in this game.
Like personally I think all tank job should have their self sustain absolutely gutted, it's the main reason I have nothing to do as a healer, the one guy who I should be healing a lot does it himself, hell they can even clear TOP with 0 healers because of it, how anyone thinks getting 2 more dps buttons on healrs will solve that is just beyond me.
Buuuuttt, if they removed it you're gonna see way less people play tanks because it'll reinforce the ol' mmo stigma I mentioned, which is what the devs seem to not want.