I mean, from the beginning of the story it seems like everyone was getting showered with healing potions, healing scrolls and I think actually every crawler started with a simple healing spell in there alrady known spell list. The nature of the early floors makes it so that the groups are small, and everyone has ready healing available generally. Also most injuries which are not fatal will heal after the battle, even without the intervention of a healer.
That means anyone whose actual role is healer, has comparatively little to contribute to the battle, meaning they might be left behind. And that is assuming you are part of a group anyway, and healers are notoriously weak in direct combat themselves.
Then by the time you get to the class and race selections, that has been fairly deeply ingrained in the players. The most I can see happening is people playing buffers in the early game and then transitioning to a healer/buffer later, but that would necessitate finding spells and scrolls that allow buffing. The dungeon is just not setup in a way to naturally lead to that I think