Why don't restaurants give you a Covid test before you can enter?

Why would they? They aren't regulated to do that, so they won't spend their money on it.

And if they did, many people would simply go to another place that won't make them jump hoops just to get some food.

COVID isn't popularly considered dangerous enough anymore (due to mutation, people's immune systems having some experience with it through prior infection and vaccines, and availability of useful treatments for rarer serious acute cases). The tests also cost money and take time.

I'm kind of surprised that they didn't do so in the past, when COVID was more dangerous and more people were more concerned about it. Possibly vaccination status was then thought to be a good alternative for reducing transmission risk, and many restaurants were legally required to check it.

Spend money on delaying and turning away and losing business from people* who unexpectedly find themselves to test positive, even though that will not prevent all (possibly most) transmission risk on site?

*Many of them would blame the restarurant, or indeed vilify it as an matter of reflexive politics?

Why should they, and why would you want it?

Who wants a swab stuck up their nose every time they want to enjoy a meal?

I can only assume that you are not a restaurant owner.

Doctor - Please don't stick Qtips into your ears. You can damage your ears.

Doctor - It is very important that you stick a Q-tip up your nose every now and then. It is very necessary in the fight against Covid.

PS. How much do Covid tests and whose has to pay for them?

Before they don't want to lose the customers who would think it's stupid,and they don't want to throw money through the window.

Doubtful that restaurant owners would want to entice the customers that testing would bring. The current customers would go elsewhere.