Because it makes it easier to talk about things with precision. We could replace all English nouns with the word thing but how useful would that simplification be? The design space of hybrid bicycles has become so broad over the years that by itself it is not much more useful than the word thing would be. So we often qualify it as in performance hybrid, trail hybrid, etc. It could be argued that some of the recent "pure" designs are really hybrids that have found a niche of their own. Cyclocross bikes are hybrids between road and off road bikes and 29er's are hybrids between mountain bikes and 700c bikes to name two examples. Manufacturers seem to be dropping the term hybrid and replacing it with a complex collection of model names unique to each manufacturer. Is that an advance in terminology? Hardly. Hybrid is a good collective noun for the beast and while we might replace it with another we would gain nothing and the word hybrid is appropriate in ways that most other names would not be.

Ken