Because it's impractical, and creates a whole batch of new problems. If you were an officer, why would you bother stopping anyone at all with that kind of liability? Now crime rises and PDs push quotas. Cops don't want to pull over anyone that could be a threat, so they pick on women, the elderly etc. to fill their quotas. Can't fuck up that pension......
Civilians are responsible for themselves and themselves alone. Law enforcement is responsible for enforcing the law. It's their job. Civvies and LEOs aren't interchangeable at all.
Look ahead a little bit....
If police didn't pursue people that slam the gas and run, more people would speed away with the understanding that police won't pursue them if they run. Now you have more accidents and more innocents injured or killed at the hands of those that are fleeing. Most likely, the person is fleeing because they have warrants or something in the car they don't want the 5-0 to find. Now we have more degenerates on the street thus increasing crime rates. Who knows what damage they'll cause.
Stay pragmatic about this stuff. Hate on the police all you want, but these proposals are short sighted.
Crimes still rise, police are over all worthless, look at the last couple years lol
But if your stop is just, you know your stop is just, you got nothing to worry about, if in doubt, just keep on driving.