Good point about the opportunity cost, but I think two points are worth keeping in mind:
1. The opportunity cost is lower than people think. You'll hear "If you make $30 an hour, it makes more sense to eat out to save time" except very few people will actually earn an extra $30 by spending less time cooking
2. Eating out saves WAY less time than people usually assume. In fact, it's probable that there are people who spend MORE time eating out than they would cooking (though clearly not everyone)

There's also the notion that cooking is a good way to help you unwind. Especially of you're a knowledge worker. It's good for your soul to make something in the physical world (and as a bonus, to get to consume it immediately afterwards)