Enemies are bullet sponges too.

That's kind of intentional and part of the design of MOBAs.

In the early game, this makes it difficult to get kills unless the enemy makes a mistake. You want players to have to choose between spending their time harassing their opposing player or farming minions.

Later on, the bullet sponge effect makes for longer, more interesting fights. That can end up, being not the case, though, if the economy is too one-sided. That's another reason for the bullet sponge design - to make the economy more important. That's how many souls you collect. Buying more means you're stronger with items and the fights are more in your favor.

So, essentially, the bullet sponge amplifies the more interesting/complex elements of the game. They wouldn't really be possible if you could just one-hit your opponent from base level.