The reason it's not called vertex program is because the pipeline is defined in terms of stages (vertex -> tessellation control -> tessellation evaluation -> geometry -> rasterizer -> fragment). Together they are a program. When talking about this structure the choice that OpenGL made is:

"A shader program consists of a vertex shader (VS), tessellation control shader (TS control), tessellation evaluation shader (TS eval), geometry shader (GS) and fragment shader (FS)."

You could instead say this, but it would be confusing:

A shader program consists of a vertex program, tessellation control program, tessellation evaluation program, geometry program and fragment program.

And it would even get more confusing if you drop the first shader:

"A program consists of a vertex program, tessellation control program, tessellation evaluation program, geometry program and fragment program."

So for the sake of it being easy to talk about, a (shader) program is the whole thing, whenever somebody talks about a "program" it's the whole assemblage. And when somebody talks about a shader, it means one of the programs tied to a stage.