...and if they are removable, why would they strike the entire set, and leave those behind?
Because Real Time covers up the white floor with black tiling, so it’s not necessary to remove the G-T marks. If a different show used the white floor, they would pull up the marks.
The less they get moved, the less likely they will be damaged during a strike.
I would seriously doubt that there's any regularly-used sound stage in the country that would have a polished concrete floor. It would be way too costly to repair from the usual wear and tear plus any accidental damage that might occur.
Most studios where I have worked do have a concrete floor, although they sometimes add a special sealant on top of the concrete. You’re right, though... they are susceptible to damage and cracking and generally need to be filled and resealed every few years.