"Naming things", one of the 2 hard problems in computing. In other words, not worth scratching your head over, ever. "Historical accidents" in terminology just such as this one abound all over the tech world. What you say specifically about "the term shader" has been true at least ever since the first consumer GPU offered a programmable vertex pipeline and their marketers chose the term "vertex shaders". How long ago was that, nearly 2 decades I reckon?
Could've been worse. Since Java was so big back then, too, we're lucky they didn't come up with eg. "Rendlets". At least "shader" is freakin' aesthetic =)
Not important to your point, but for historical flavor--
I'm pretty sure the term "shader" was introduced by Pixar, from way back before Toy Story when they were doing little demos and short movies. They were building very modern-looking graphics pipelines in software way before the first GPUs (I think Nvidia coined that term?)
Could've been worse. Since Java was so big back then, too, we're lucky they didn't come up with eg. "Rendlets". At least "shader" is freakin' aesthetic =)