Coinbase is meant to serve a more retail type of customer who doesn't know much about trading and is really only interested in making a long term investment with the highest level of security and guarantees possible with the least amount of hassle. If you're that kind of customer, the fees really aren't that onerous.

On the other hand, GDAX is aimed at the more sophisticated short and intermediate term traders who probably keep a nice proportionate longer term stash in their Coinbase wallets as well.