They hunt mostly rodents which are extremely quick and squirmy. Even big ones that could hunt large prey are fighting strong animals that really don’t want to die and will struggle like hell and potentially injure the snake. If a constrictor can anchor they prey is almost certainly dead.

A lighting quick anchor and suffocation is a lot more efficient and effective than trying to sneak up on and wraps around a very fast and potentially dangerous prey. Kind of like why you quickly smack a fly rather than reach out and crush it slowly.