Is graffiti also freedom of speech? If i have a wall does it mean i can write whatever i want on it? I guess not. These are social norms. And we could decide that advertisement is not ok in public spaces like it's not ok to walk around naked.

Not an equivalent situation at all. Graffiti makers don’t have billions of dollars to hire lawyers that challenge laws that ban advertising. Society increasingly runs less and less on social norms and more on laws.

Graffiti is illegal because it involves damaging property, not because of its textual / visual content...

But if i had a wall (own), could i write anything i want on it?

Plenty of US cities and towns have regulations prohibiting billboards.

Interesting, I didn’t know that. Maybe it would be possible legally then, although I imagine that NYC would be a tough place to get it passed.