You don't declare a no fly zone you enforce a no fly zone. In other words you shoot down anything that flies in the designated area until nothing flies anymore except the people enforcing it. I'm sure if Ukraine could do it they would. If the US and our allies do it, and we start shooting down Russian planes, that would mean direct involvement in the conflict and that would mean that any of our military assets would be fair game for the Russians, which could very well trigger WWIII. I don't think any of us wants that.

Of all the stupid crap the Biden administration has done, avoiding direct conflict with Russia is not one of them.