Why is Starbucks Brewed Coffee so high in caffeine content?

Was looking online and saw that

McDonald’s Caffeine 185 mg - 21 oz

Starbucks Coffee 310 mg - 16 oz

Dunkin 270mg - 20 oz

Starbucks is Quite Higher. Any opinions on this? Is that why Starbucks is popular, due to high caffeine levels? My stomach gets an ache when I drink Starbucks.