One of the reasons for that is that safety requirements are constantly changing. What is even more interesting is that there are no legal regulations as of yet which would require companies to set an expiration date. Nevertheless, it is a common practice to do so.
There is another reason for that, other than new designs coming up, that is. Plastic may be a good material to make car seats of, however, it is capable of fatiguing, much like metal. Extended use leads to cracks developing and, eventually, the seat may become useless in the event of a car crash. It is better to buy a new infant car seat if you have another baby rather than pass the old one on.