I wonder why OpenGL is not capable if doing transparency itself. Do you know why? It would seems so simple for a microchip doing all kinds of complex stuff!!! Why give us the burden to make complex algo that can't be perfect, and waste time on sorting when we could be computing something else that's more important for the game or whatever it is!
To answer your question on a higher level:
Hardware manufacturers went down that path once before, with the fixed function pipeline. You had a fixed set of features the hardware could handle, and that was it. If you wanted anything more innovative, you had to do it in software.
It was soon very clear that this was not going to work. Instead pixel/vertex shaders became the norm. Then multiple rendeer targets, instantiation and hardware tessellation, etc. Graphics hardware is now moving in the direction of becoming a completely generic CPU, optimized for extremely parallell tasks, such as graphics, but also physics, password cracking and all kinds of number crunching.
This means we are not limited to what the hardware manufacturer had in mind, but can invent new techniques. The only drawback is, you have to do more yourself...