Well, no offence, but you idea of how society works is not particulatly correct. Both individuals, and institutions have to prioritize thousands of issues against limited resources. Because of that majority opinions doesn't necessary become policies. Only those urgent/emotional/tribal enough to become election fuel. Apparently, graffiti cannot compete with plenty of pains citizens experience now.

>A lot of cities even give space to graffiti

Try researching where it came from, and you'll see it's an attempt to civilize behavior cities found impossible to control.

So reality is that graffitists are active, and numerous to extent it's hard to fought them off the walls so to say, and while majority doesn't like it, it's not ready to re-allocate resources from other issues. This leads to equilibrium we are in at the time.

>plenty of gray boring

It makes a good excuse in the internet discussion, but it doesn't correspond to street reality. I'm in a nice medieval quarter now, and see lots of graffity across buildings which neither gray, nor boring. It is as if people who do that don't care about beauty, other humans, and all those good things usually claimed to legitimize the phenomenon