Why didn't Tom simply open the window instead of breaking it?

Tom and his wife lived in an old apartment building in Manhattan. The windows in most of these buildings were what were called double-pane windows. They had counterweights inside the wall which were intended to make them easy to open and close. There was a cord and a pulley and a lead weight. The cords in those days before nylon were made of cotton. Over the years the cords would deteriorate and eventually break. Furthermore, the window sashes would get painted and repainted and repainted so many times that the windows might get completely stuck or else would be harder and harder to slide up and down in their tracks. 

He couldn't open the window. It had been pulled not completely closed, but its lower edge was below the level of the outside sill; there was no room to get his fingers underneath it. Between the upper sash and the lower was a gap not wide enough--reaching up, he tried--to get his fingers into; he couldn't push it open. The upper window panel, he knew from long experience, was impossible to move, frozen tight with dried paint.

The only way Tom would open his window was from the inside. He had to hit it with the heels of both palms in order to un-jam it. His wife couldn't open the windows at all. This is what happens in old buildings. The landlord doesn't want to spend money on repairs and the tenants are afraid to complain because the landlord might raise the rent. Eventually, the land under the building becomes more valuable than the building itself, and the owner is only biding time until he has the existing building torn down and replaced with a modern structure, which could contain apartments, condos, transient hotel rooms, or office suites. In the meantime many landlords "milk" the buildings by charging the maximum rent and doing the bare minimum of repairs. Deterioration is seen everywhere, including in the carpeting in the hallways.

When Tom was on the outside looking in, he couldn't hit the upper part of the window with his palms because there was no place to hit it on the outside, only on the inside. That explains why he had to break the glass.