I'm fairly sure there's a hard limit to what type of lens can be made. So for example, take the EF 50mm 1.0, there's not a massive amount of glass for light to pass through, so do-able.I am genuinely keen to understand the limitation. Any hints?
But the longer the focal length the much more massive the lens and a lot more places are required for the light to pass through and bounce around before it hits the sensor. The closest we've got so far (in mean terms) is the massive 1.8 200mm prime but even then the next revision to that lens was F2.
Physics will also play a part in the resulting image quality so gets in the way of a lot of things. We didn't get a 50mm F1.0 mkii as another example.