Suppose your lawyer thinks you have a 90% chance of winning - against a neighbour who might run up a $1000 bill for legal fees, the same as you. Then it's worth the risk.

Now suppose you are up against a bank that can run up $10M in fees (or claim their in-house lawyers cost that) - now to be $1000 at risk you need a case that is 99.999% sure before the lawyer's insurance company will let them take it.