I tip on take-out - every single time, but for pizza. We've covered this here before, but I'm too lazy to look it up.
That bag of food didn't just appear. Someone took your order, made your salad (without onions, extra tomatoes, and dressing on the side), packed up your dinner so it wouldn't spill, packed your utensils, napkins and sauces. Then they checked that your order was complete. THEN when you received your bag. There is also a lot of back and forth about who has to work those shifts and what they get paid for doing so. Those servers are no different than the ones that wait on your sit down dinners, other than performing less per customer for more customers.
So, I tip $2 to $4 on my takeout. It's not 15-20%, but I didn't receive 15-20% service. However, I DID receive service and quite often by people paid as wait staff, but not getting a normal shift to make their tip money.
Argue all you want for your personal cheapness, and argue you will, but try doing a search or two about this and maybe you'll think differently.