In a blockchain, the chain keeps splitting. Your tokens live on multiple competing chains simultaneously. In PoW, you are forced to pick one of those chains to mine on. In PoS, you can stake on all of them simultaneously.

The splitting is unavoidable and happens constantly. Multiple competing future states are constantly being created, and the network has to eventually arrive at a consensus about which possible future is the true one.

> Where is the difficulty in making miners expend their tokens (i.e. in a way that is irrevocable) instead of merely depositing them somewhere?

Figuring out how to make spending your tokens irrevocable is the whole point of PoW/PoS. Your question reads to me like "In trying to solve problem x, why don't you assume that you've already solved problem x, and use that to solve problem x."