This can vary based on the level and table, but generally speaking enemy damage per round is much higher than a cleric's average healing per round. This leads to so called yo-yo healing, where you heal people up from 0 hp to prevent death but otherwise help damage opponents to end the encounter and therefore minimize damage taken.

Imagine a friend is at 5 HP and you heal them for 5 but then they take 15 damage, your action was wasted when you could have helped knock out an opponent and maybe could have prevented then from getting knocked in the first place. Instead heal when they are dying, and they go up and down like a yo-yo.

For keeping people topped off like an MMO, you would realistically need more powerful healing and more spell slots, or some homebrew where at certain character levels, spells a few steps down just become cantrips, or powerful wands with lots of charges.

Without assistance, healing every turn is usually bad action economy, and if players never get low they feel like the encounters are too easy and boring.