The gaps behind the W/D are pretty much necessary but not sure why the gap behind the sink but maybe it was done to keep the sink more forward for use.

This is common for everyone. I would do a shelf along the back, which thankfully you can since your sink is also pulled away from the wall.

Might be hard to a shelf with your tall faucet

*I'll clarify...a shallow shelf, about 8" that fills the distance from the wall to the w/d and sink faucet.