Because they are expected to. Mostly, he says, they police themselves. Regardless, they are continually marshaled to make sure they don't fall behind. Marshals aren't gruff. Rather they try to help the players - looking for lost balls, raking bunkers, politely informing them they are X number of minutes off their pace.
Nobody likes slow play.
And, of course, nobody likes being adrift in a yardless world, so Kirby markers make the experience predictable and swift. Golf remains a straightforward game. Find your plate, hit your ball.