The challenge is one of delta-v. To get from any orbit to any other orbit requires a change in velocity which requires propellant. Space is notably empty so you can't just go harvest yourself some propellant.

The ISS is in a particularly crappy orbit with respect to servicing satellites so a repair ship docked there would require more propellant than one in a better orbit.

The ISS is a nice destination for people because it's got consumables and room to move around. It's not a good pit stop on the way to somewhere else though. There's no spare propellant stored there so the only reasonable place to go after visiting the ISS is back to Earth.

You could launch another station with blackjack and hookers in a better orbit but there's still the propellant issue. Every time you'd want to service something you'd need to send more propellant from Earth. Just sending a dedicated vehicle up that could do servicing (like the X-37) would be far more efficient.

Or possibly a dedicated servissing platform, say with ion engines or refueling capacity. It could do slow orbit changes unmanned with crew launched only for the actual servissing, or possibly dragging broken sats for repair to a manned station and back - could be pretty workable in GEO belt if you manage to get and sustain a crew there.