I would imagine because we don't all pilot our vessels well enough to avoid the occassional collision? I know I've clipped enough asteroids in my day. I have nothing against collision rules, but they don't seem to be neccassary to me.
Back in the day Klingon Academy had collision damage, which lead to many a waiting game while the crew repaired the damage caused by getting a tad to close to the enemy ship and having to press the right key sequence for full stop or emergency reverse.
Of course by that time my BoP and the enemy ship met with such a CLANG! noise and it was too late for full stop or emergency reverse to take effect...
Now for those of us who have large cruisers... Those suckers don't exactly stop on an energy credit... And all that mass flying into another object would equal TONS of hurt.