MOST animals operate on the principle that, when it is not necessary to do something, it is necessary to not do anything. Gathering calories is an expensive process, especially for a large creature. Cows don't do a lot more than chew and nap either.
You need to spend energy finding food. If you're territorial you may need to spend energy on patrolling the bounds. If you're a pack animal, you need to spend energy maintaining your pack status. If you're a prey animal, you need to spend energy avoiding being eaten. If you're an infant, you need to spend energy learning to be an adult, which accounts for most "play". You need to spend energy finding a potential mate, though that may be seasonal. You need to build up energy reserves to last you through the off-season or any unusual situations that arise.
Homo sap has the advantages of (potentially) being able to store surplus uneaten food and of having pack status determined in part by things that look like play (including sex, which we've folded into our grooming behavior repertoire). Most critters have other priorities, and wasting energy is not one of them.