There seems to be two issues being discussed here. One is altitude and the other is airspeed. All airplanes eventually land and only a small percentage have radar altimeters. Premature contact with the ground is a real possibility ("When flying from a high to a low, lookout below"). To help avoid that, which is usually caused by pilot stupidity, but can be contributed to by changes in air density, pilots receive local altimeter settings. In instrument conditions, there are other aids to maintain a safe altitude besides radar; however, the altimeter is still critical until within 200 feet of the ground or so.
As for airspeed, remember that the pitot tube also has a reference port, called the static port. Air density does make a difference and can be corrected for by altimeter setting and altitude. However, since all airplanes at the same altitude and location would have the same correction for density, pilots and controllers generally rely on the indicated airspeed and forget about the corrected airspeed.