So where is free sling? Free roach speed? Free hydra speed? free range? Why not just make it so we start with burrow?
Blizzard gave reasons when they released the patch.
Go fetch it and find out.
Way to do research.
Blindly raging against it without actual reason is cool too.
Because no one uses unsieged siege tanks, they're practically useless. The only time people do use unsieged tanks are for when peoples tanks are caught out of position, for example. Other units can be used somewhat well without their upgrades like stalkers and blink, or zerglings and zergling speed, and ghosts and cloak.