Two questions about optimistic rollup

Let me first list the two questions that bothers me recently:

1. Why does it needs the bisect game in fraud proof?
2. Why can the transactions in the rollup be fast?

Ok for the bisect game, I saw on l2beats that Arbitrum, base and op stack are all using bisect for their fraud proof. I can't understand why, isn't a fraud easily proven by just executing all the transactions and provide the needed l2 state? If that's too much gas for a single l1 block, the challenger can split it themselves, right? I don't see why it needs an interactive bisect game.

For the second, I know that withdrawing to l1 needs several days for the state to be challenged. However, why isn't it also apply to transactions within the rollup? If I transfer money to another account, and later that block got challenged successfully, then my transfer doesn't count, right? So I don't understand how my transfer can be settled before the block itself got settled.

That's all, thanks in advance!