I like to imagine healing magic as almost a targeted time reversal. Otherwise, things get would get strange when people are overhealed.

My head cannon is also that repeated magical healing increases the risk of cancers and defects when it is shown as a form of natural regrowth. It’s totally worth using in combat if the alternative is death, but you risk shortening your life with repeated use. This helps explain people having sick battle scars and other wounds in most fantasy settings.

The other explanation that would make sense is that healers are exceptionally rare, but since we are heroes in these stories/games, it just doesn’t seem like it to us.