It's because it shows Indicated Air Speed (IAS), which is derived from Calibrated Air Speed (CAS) rather than true air speed. So the thinner the air (higher altitude means thinner air) the slower your IAS. IAS isn't that useful if you're thinking of speed as you would if driving a car (ie. ground speed).
That's it in laymen's terms, I'm sure someone else will explain it a lot better.