Trust between people is difficult to be built and easily can collapses. It is normal in bitcoin world because it is trustless when there is a transaction between two parties, and mainly when this is the first transaction between them or the amount is big enough any or both of them to ask for an escrow so that the transaction to be smooth. Sometimes it's an easy task but other it's not. You can find some cases in the forum where there is no agreement between them. Therefore an incentive motivates people who are escrow to do properly this process.