Many investment term sheets mandate it. If you don't want to vest then don't take money from someone who says you have to vest.

Operators always vest. Nobody would give your firm money if you could leave the day after the wire cleared.

You can of course say no, but nowadays it's so common that you are unlikely to find someone who would make a deal without it. Basically it goes to show that the people who have the money are the ones who have the power, not the CEOs of start-ups.