Why do ski lifts close so early, even when there are still daylight hours left

Mountains base their hours not on daylight but on customer response and employee payment. They close at the time they can get the most money.

To give the ski patrol a chance to throughly sweep the mountain and make sure that no one is still on the mountain or hurt or anything like that.

Resorts want people off the trails early enough that they can have instructors or safety people make a last pass to find injured skiers or stragglers, and to do any maintenance work needed on grooming equipment or snow making machines.

At Wolf Creek, CO, they extended their lift hours to 5 pm for the spring daylight savings time. Some resorts have lights for night skiing, for an additional fee, of course.

Because your resort sucks. Haha, just kidding :) Its probably just either small, or on the East Coast.
l am in the Pac NW, and my resort stays open until 10pm.