Why has my Command Module's orbit not decayed?

On one mission to put Bob Kerman in orbit of Mun and bring him back, his rocket ran out of liquid fuel on the return trip. However, the orbit Bob ended up in was around Kerbin with a periapsis of less than 70 km. So, I figured the atmosphere should gradually slow him down until eventually he'd end up ready to land on his own. Thus, I went on about my business with launching other missions and would just occasionally check in on Bob to see when it would be time for him to come home.

90 in-game days later, he's still not home and his orbit really hasn't dropped all that much. I've tried accelerating the process with Time Warp from the Tracking Station, but his orbit doesn't change at all there. It only appears to get adjusted for aerobraking when I'm actually focused on Bob's craft. Then, the Time Warp is very tedious since it automatically adjusts down as altitude drops and won't go faster than 4x when in the atmosphere.

Is this just a limitation of the physics engine in KSP - that certain factors such as aerobraking aren't taken into account for objects that aren't explicitly focused upon? Or is there something else that might be preventing Bob from coming home without me having to watch him all the way?

The orbit Bob is in currently has the following stats:

• Periapsis: 61,993m
• Apoapsis: 1,751,570m
• Inclination: about 20 degrees
• Period: approximately 1h 43min

P.S.: Sending someone to retrieve him is not an option at this point in my game.