Healer is in need because they didn't add a Healer since 3.0

[Taika]

And healing is objectively more difficult than DPS because you need to be aware of your own character in addition to the rest of the party while still executing the same mechanics. A bad healer will always wipe the party but a bad DPS can truly get carried throughout almost the entire game.

Being a passable DPS (being able to beat the DPS requirements in extremes and Savage content) is significantly more demanding than being a healer who's managing to keep people alive and not much else in that same content.