the TCP stack is controlled entirely by the kernel. so it handles things like requesting dropped packets and rate limiting.(which relates directly to "packets i

the TCP stack is controlled entirely by the kernel. so it handles things like requesting dropped packets and rate limiting.(which relates directly to "packets in flight") with UDP, that layer of ACK, ordering and rate limiting is gone. if you want to send a million packets a second to 56k modem, sure the kernel isnt going to stop you. Whilst it might be possible to have per TCP connection windowing parameters, I doubt it'd be very efficient.