It's not. Because they're all doing extra damage, they just get it a different way. It's not the same as how sorcerers can't have good spells because the wizard's 'thing' is having good spells, or the whole stupid arcane healing taboo from back in the day.
Back in the day, having healing spells on your list was a burden, your spells were going to get sucked up as healing or your party would die. No healing meant you could cast a wider variety of spells.
"We need a Cleric" was a classic refrain from the olden days, the concept was unpopular and the bandaid function unappealing. Subsequent editions tried to address the problem.
1e gave the cleric bonus 1st level spells for modest WIS, which typically all got used to heal.
2e added more-powerful specialty priests
3e gave clerics more spells than ever, extra domain spells, and the ability to cast healing spontaneously using slots in which other spells were prepared... and, perhaps inadvertently, WoCLW... the result was CoDzilla
4e moved the healing burden to the character being healed as surges, re-labeled the healer role as Leader, and made basic healing a minor action. It also provided a leader role class in every source, even Martial, so if Cleric, Shaman or Ardent didn't appeal, maybe Bard, Artificer, or Warlord would.
Oh, look, accidentally back on topic, the Warlord filled the support role adequately, enabling functional parties without supernatural/magical powers.
5e nerfed surges down to HD, made in-combat healing bad, and "eliminated roles." It also cut the Ardent, Artificer, Shaman, and Warlord, but added back healing to the Druid, and, later, the Artificer.
Making healing a generally bad idea in combat
did reduce the healing burden on the cleric, tho. Instead, play apparently tends towards short days.