Answer: because (1) not all who go to a bar drink, or if they do, do not drink enough to become impaired, (2) those that do drink too much may be there with a designated driver who doesn't, and (3) employees, suppliers, and others also need to park there.
The problem with both the DUI and the gun debates on both sides is that people tend to resort to statements, not unlike the one you just made, that are meant to get an emotional response rather than looking at it more logically. I hate drunk driving. Four members of my family were killed and another injured all in a single accident because some idiot drove while seriously intoxicated — like falling down drunk — and should have known better than to drive. The drunk driver of course survived. As this was in the days before DUI was taken seriously, the drunk driver got a slap on the wrist. That isf part of the reason I have over the years supported many bills to crack down on DUI driving. But I don't get behind the more extreme ideas. All regulation should be reasonable and target as narrowly as possible what the real problem is. You don't use a nuke to swat a fly.
I take the same view of guns. I support the 2nd amendment but I also see a place for reasonable regulation. I have no respect for the at least some of the most ardent gun supporters that all regulation affecting gun owners must be opposed. That unfortunately seems to be the prevailing view of the NRA in the last several decades, even though it did not always hold that position. Just like with cars and DUI drivers, some firearm regulation is needed, but it should be reasonable, not overbroad, and target the problem we seek to address.