It has to do with the type of compressor which is used. The system in a refrigerator/freezer has a completely closed unit, where the compressor is housed inside of the gas. Because of this, all the lines associated with it can be soldered shut with hard lines, etc. Because of this, a house refer/freezer will not cool as quickly, either. They work very well, but you also have insulation and a lot smaller volume to cool.
A car's compressor is housed outside and the refrigerant pumps through it, with lines which are made of rubber, and seals which deteriorate over time. As long as the seals and hoses remain in good stead, the car's A/C usually will continue running without issue. Lot's of times it's lack of use which causes A/C equipment to go bad. Vehicle A/C units cool a relatively large volume of air (as well as everything in it -- such as seats, dash, glass, etc.) in a very short time.
We expect much more out of a car's A/C than we do out of a refrigerator. They don't stand up as well because it is made to work and work fast, which has the effect of wearing out faster. If you don't want the car to cool as fast, I'm sure they can make one which will last longer.