Those who would fault the line manufacturers for making such a wide variety of lines generally have a common trait: They fish one way for one species in one geographic area. If all I did was fish for trout in mid-sized streams, the standard WF or DT lines of 20 years ago would work fine for me.
But the world is a big place, and many saltwater anglers, for instance, need fly lines that are designed for 85 degree water; the coldwater trout lines behave like over-cooked spaghetti in warm water.
A guy throwing Clousers or bass bugs really benefits from a torpedo-type taper; someone on a spring creek will catch more fish with a finer presentation; Spey casters almost have to have a specialized line, or their method is futile; beginners may want some choice in line costs; and the different sink tip weights make matching different the water depth to the fly line much easier.
Sure, the variety forces the single-use angler to do some reading and ask some questions before he chooses a new line. That's the price of progress.