Well, fortune has it Friends did the right thing post-9/11 ... and in the years after too. E.g. 2003-04 to "restore" the towers to the screen eventually.

It would have done no justice to the victims, the lives that went on and to the spirit of that day if we were gonna pretend this huge symbol didn't exist.

I'm not sure why the towers are still absent from season 8-10 though. I don't think it makes that much of a difference. Not to forget, with the spirit of the show in mind. For being a source of "relief" and a form of escapism. E.g. with the sudden absence of the towers in season 8-10, being a constant reminder of 9/11 until the very series finale itself.