What is "spherical keycap" (or "spherical family" of keycap) and why is it so difficult to perfect its manufacturing process?
Spherical topped keycaps are what keyboards prior to 1981 all used. They look like this:
Today there are only three spherical keycap families being made for new keycap sets:
1. SA from Signature Plastics
2. DSA from Signature Plastics
3. "Hi-Profile" from Topre
There is a beamspring clone being worked on somewhere in Asia, but it is currently vaporware.
I don't think spherical keycaps are inherently more difficult to manufacture, it's just that nobody but SP and Topre have spherical molds anymore, and there is no large-scale market potential to propel new companies to bother. Catering to the niche custom keycap hobby is left to Signature Plastics who inherited their spherical molds from the company they absorbed way back when they got started.
What appears to be difficult is double-shotting PBT plastic. SP and Topre only dye-sublimate onto their PBT sphericals. Vortex sells double-shot PBT keycaps, but they are not highly praised and they are cylindrical not spherical.