Because all hunting laws are (or should) be to either a) keep people safe or b) to give the animals a fighting chance, to keep it more "sporting". There's a lot more to hunting than just killing animals, in fact that's a tiny percentage of what you're out there doing (scouting, tracking, hiking around, sitting still for hours, etc). All of the "sport" part of hunting goes out the window if you can simply spotlight deer after dark and knock em over.

And if you wanted to know the REAL reason these laws exist, it's the same reason that gyms over-sell memberships. In many places, including where I live, the state wants your money, but don't want EVERYONE with a license to get an animal, because that wouldn't be sustainable. There are places where more tags are given out, and special hunts are provided to do actual population control, but I'd wager that a lot of states don't have that level of problem, and simply want the cash.

So there ya go. It keeps the "sport" of hunting alive, and generates more revenue for states because they can sell a ton of tags that don't get filled.