Mostly because car manufacturers don't all own gas companies, or have exclusivity agreements with them.
No, really. If they did own gas companies, they would lock their own cars such that they could only use the right brand of gas, and then they wouldn't care about how much fuel got wasted since any drop in customers due to bad MPG would be made up for by increased fuel sales. Then we'd have reason to hack in, to remove the station-specific locks, and to correct the fuel injection amounts.
As it is, it's in the manufacturer's best interests to get those as good as possible on their own, and they do a near-optimal job. Nothing to fix.
We'll see if this continues once actual computer interfaces in the passenger compartment become commonplace, however. If they decide to try to prevent us from adding our own applications and similar, I'm sure we'll see just as much jail-breaking for cars as we do for phones.