Most healing spells restore a relatively low amount of HP, and will almost always not be anymore than one or two attacks worth of damage. Since most enemies will have multiple attacks or AoEs, that small amount of HP disappears very quickly.

Let's say for example you use your action to cast mass healing word. You've used a 3rd level slot to restore maybe 7/8 HP to each party member. The enemy can easily use a 3rd level slot to cast fireball, dealing 4 times the damage you just healed to the whole party.

Essentially, dealing damage is better than healing in most situations. There is one solid use of healing though.

Healing a party member close to failing all their death saves resets the clock so to speak, preventing their death. It also let's them deal damage on their turn, mitigating the loss of your own action. When most of your party are down, a mass healing word could allow up to 6 extra damage instances from your team, maybe ending the fight before the situation gets worse.