Have you ever had a hamburger on a bun without sesame seeds on it? Did you miss the sesame seeds?
It's not like the sesame seed doesn't have other, more useful places to be. Like in sesame oil, or tahini. It's not some bullshit vanity grain, like the poppy seed.
There's probably just some dude who owns a bakery and has a bunch of sesame trees in his yard. This guy is like, "What am I gonna do with all-a them sesame seeds that fall outta them sesame trees every fall? Fuckit, I'll put 'em on the hamburger buns. Charge an extra 35 cents per dozen. Then I write the trees off as a business expense. Yeah, screw the taxpayer, that's the idea!"