I’m not sure I entirely agree with the OP, but they are different. I’m going to try to link to an old post of mine:
Most fast-food restaurants serve sliders. One thing I didn’t note in my post above is that sliders are usually cooked on a flat-top or griddle which doesn’t char the meat and doesn’t have a “taste”. Whereas your backyard BBQ is flame (either gas or charcoal). Charcoal imparts a strong taste itself, but even a gas grill picks up a “flavor” over years of cooking - especially if you don’t clean the flame diverters or bottom of the grill after each use.
I personally prefer a burger cooked over flame, but there are times when only 5 Guys will satisfy the craving I have (much like I occasionally NEED Taco Bell, and only Taco Bell will do).
@Two_Many_Cats2 - BK is unusual in that it cooked over open flame. It could be you just prefer a flame-cooked burger.