Fuel injected motorcycles don’t have reserve (at least, none that I’ve seen.) instead they have low fuel lights or full fuel gauges. I’m guessing it’s because the fuel pumps are in the tank and the fuel injection system needs high pressure.
Fuel injectors require filtered gas because even small particles can clog them, and said filter is more likely to be clogged or even compromised by sucking up the last drops of fuel (and scale and debris) in the tank, so the low-fuel warning is required.
Carb jets can get clogged, too, but are wider since they're not under as much pressure. Also, since they're a wear item they're a lot easier to clean and/or replace.
I think grandparent commenter had it right: it's because the pump is in the tank. There's just no good way to have an external petcock determine where a tank-internal pump gets its fuel from.
