It's not complicated, if you can't afford to leave a whopping $4 gratuity for a $20 meal, then don't go to a sit-down restaurant and take your ass to McDonald's.
Imagine spending $200 on a meal and not leaving a tip. You can afford to waste that much money on a meal but you can't afford to tip?
well to be fair it does add up after awhile, all that tip money you use could eventually equal another meal that you could get if you didn't