why raid5 not like this?

in linux raid4/5/6  ,one big bio will span several devices,and There
may be several bio's per stripe+device.if the bio arrived in bi_sector
with overlap,we must wait the former bio to be finished.
why not we split the bigger bio into small ones?we can attach the
smaller bio to the stripe+device (here we may can do some merge ).one
disk i/o we can finish many small bios.if all the small bios of the
request be finished ,we can return the request.