It's less about the difficulty of the process, more about efficiencies. I don't have time to find numbers, but I believe it's a pretty lossy process to split water, and then turn it back into electricity in a fuel cell type device.

There are also (probably) a lot of costs involved with trying to contain the hydrogen, and making and industrial sized fuel cells. Maybe quite a few unsolved problems as well e.g. do fuel cells eventually wear our like batteries?

Basically, we could do it, but the current techniques are possibly more expensive and too lossy compared to other options, like molten salt, pumped hydro etc.