I suspect that lifestyle and financial reasons may be the main motive for moving, for those that wish to do so. Especially if you are single and don't live in the SE and commute to LHR.
What makes people think the take home pay at BA is worse than Emirates? That would all depend in which tax jurisdiction you
choose to live. A significant number of BA pilots are non-resident anywhere for tax purposes. That's comfortably workable on any fleet! Their take home pay is very impressive!
Looked at in that way, other than an ego-command, BA wins hands down doesn't it?