What people usually mean when they say this is simply that college is a fantastic experience - not that it's going to be the only one in your life.
The combination of being young, unencumbered and part of a huge community of people who are looking to have fun means that it really is easy to have a fantastic time. If you aren't enjoying yourself, it's more than likely because you simply aren't making full use of what's available to you.
Obviously not everyone likes to get out and network, but most people do (geeks included) and so for them college can be a great experience.
Basically, "enjoying life" is entirely something you choose to do. If you aren't taking the steps to make college really fun for yourself, then it probably wont be the high-point of your life. Presumably you'll discover a happy lifestyle somewhere further down the line and enjoy that instead.