Why do burrito bars force you to “bundle”?

I find that most places like Qdoba and Chipotle (and similar places) usually have tomatoes mixed with onions (“pico de gallo”) and peppers mixed with onions (“fajita veggies”). I love tomatoes and peppers, but since I hate onions, I can’t get them since they’re pre-mixed.

They already have separate ingredients on an assembly line, which they mix on the fly. Why can’t they just have tomatoes, peppers, and onions (all separate)? And even if they really can’t add one more tray to the line, they could still just have one tray of peppers and one of onions - I’d still be missing out on tomatoes, but at least I could have peppers.

Moe’s is used to be great about this, but they left my state.