At least as of a couple years ago, the top floor was occupied by the building’s owner (Goldman Sachs). Presumably they are happy to keep the view for themselves.

Also, a public observation deck would require a couple of dedicated elevators to keep the public and the office tenants separated. Since you can’t easily add new elevator shafts to an occupied office building, the only other option is to take away elevators from the existing occupants. That would be a pretty big deal because companies in a building like that don’t want their employees to waste time waiting for elevators.

And they would probably need to build a separate lobby for the observatory for security reasons.

It’s not impossible to do, but I imagine the costs would quickly add up.