The point of fees isn't primarily to reward the miners. That's what the block reward is for. Under PoS validators will still get a block reward, although a lower one (90% less).

The fees are to mitigate spam attacks. You can only fit that many transactions in a block. Fees are bids to be included in a block. The higher the fee, the earlier your transaction gets included. Fees on Ethereum are high, because a lot of people want to use it.