Wakanda is a well oiled machine, and doesn't necessarily need their leader there 24/7. Besides, it's the same deal as Aquaman being in the JLA. They protect the planet (at least they're supposed to), and since Wakanda is part of the planet, it's kinda something he should be at the very least paying attention to. Also, and this is the key to why super kings, space cops, and other people who have seemingly more important day jobs can convincingly spend their precious time messing around with other costumed do gooders, the average adventure last anywhere from a few hours to a couple of days. With a super sonic hyper speed jet, BP can help put the smack down on Ultron, and be back to stamp some s**t with the Wakandan seal by Lunch.