Reason for "the" as whole class

The construction you ask about is called a Generic noun phrase; this kind is a Definite Generic.

You should know, however, that there is no single rule for use of articles. Instead, there are hundreds of individual rules for special cases, particular noun phrases, idioms, and syntactic constructions like this one. And each case has to be recognized and memorized separately, because they don't correlate with one another.

The and a/an are just markers, like flags of one color or another, that indicate specific usages and don't have any meaning themselves. Asking how to use the is like asking how to use the color red; there aren't any general rules, only specific ones, and everything depends on the context.