Mountain bike tires are lower pressure so this makes sense that they don't lose as much. To me, the tradeoff for decreased rolling resistance is well worth the <1 minute of pumping.

If you haven't invested in a good floor pump, now's the time. Topping off tires daily (I have to do it semi-daily, myself) with a frame pump or minipump is for people who just don't like themselves much.

I commute on 21mm wide tires and never get a pinch flat even if I run them at 60 psi. Of course, I would never run them that low intentionally, unless I have a puncture in the tread. Then I squirt in some Tufo sealant followed by whatever pressure the tires will hold until I can do something more permanent.