Why are so many films based in New York?

Seems to be a trend. New York has a cultural aspect to it that might be symbolized as American city life of industrialism/arts/history/culture or some other aspect/tone of that nature.

"The city that never sleeps."

Many media and people have likened New York as one of the "the" cities that has a major element or aspect of U.S. culture or life. Also, it being a state with a massive population squeezed in to one small region makes it have a bigger sway in ways when it comes to culture too.

The whole New York symbolism is a lot like Toronto in Canada where it is also a culturally tied place of Ontario known as "the six."

Of course these result in stereotypes which then enables people to assume all Americans or such are just like those they encountered at 'X' place and 'Y' time or such.

Basically New York has the record of being a "hot spot" in the U.S. along with L.A., Houston, Miami, etc. Since it has this major cultural and social identity to it, it oftentimes is probably used as a "stock locale" for works/fiction/etc. due to its pull/name popularity as well.

Old Blue Eyes tells it...
https://www.youtube.com/watch?v=EEjq8ZoyXuQ

That's because many tv/film writers live in New York City, so they're more inclined to write stories that take place in the Big Apple. It's also why you see suburbanites erroneously written to look, dress, and act like Jersey Shore socialites. Because that's as close as the city slicker writers will get to seeing what the suburbs are like, so they just assume it's all like that, hence the fake look to it.

b/c it's like having another compelling character in your film. it's a draw all on its own.

Great tax breaks and other perks for filming in NY and they actively court the film industry. Georgia does the same which is why so many shows are filmed there, too.

The cultural and economic capital of the USA.

The scenery and locations can be moody (the Central Park showdown at the end of Wall Street for instance) and sometimes very pretty in NYC (ariel
views of the Hudson River and the harbor come to mind)

They DO pretty the place up in a lot of movies...there is litter and dog poop everywhere and the rats are common here
And you can't walk twenty feet without a bum asking for a buck or a 'rapper' trying to sell you a demo tape on the corner

New York often looks way better on film

In the case of action movies with big battle sequences, the sheer size and the density of the buildings makes for a dramatic aftermath.