Is there a reason why BG queue does not require everyone to accept to start like dungeon queues?

LinusMain

Had quite a few matches where it ended up cancelled because couldn't find substitutes for those who did not accept the join BG offer.

Freelancer_ESO

Probably half of my matches have a minute or two delay because somebody said no and the system then needs to backfill.

Under a system that requires everyone to be ready first all of those would have been cancelled instead.