Because Scrooge McDuck diving into a vault-full of gold is a cartoon and not real. People with wealth generally want to do things with it. Buy yachts, build Casinos, hookers and blow, all that. And that requires a currency that others transact in.
If everyone is just storing their wealth with very little in the way of transactions, mining becomes less profitable, miners leave, network security falls away, money leaves Bitcoin and it all becomes worthless. Which is why that won't happen.
This is all rehashed stuff so I'm going to stop here. People can make their minds up in their own time and blocksize will increase in good time.