Others have given good answers, too, I’ll just chime in with mine.
First, texture sample operations have a cost. The hardware may pay the same cost (in cycles) for sampling and filtering a single-channel texture, as it does for a 3- or 4-channel texture. Might as well get more data with that read operation!
Separately, the graphics card can’t know whether you’ll always be using that particular texture with that particular shader. From the graphics cards point of view, you can bind any texture object to any sampler (more or less) and clearly you have some shaders that use all channels of some texture. There’s no way to pinky-promise to never bind a particular texture to that particular sampler.
Finally, texture compression generally saves more memory than trying to just trim out a single channel; the reason this is so is that material properties generally vary together, and as long as there is some correlation between each channel, the compressors can do a good job.
Packing two or three channels into a texture will give you higher runtime performance, AND less memory usage, than trying to use two or three separate texture maps and samplers.