Why do ~all mice and trackballs have keys with short travel? For keyboards, ~most custom builds use switches with large-fraction-of-cm travel. For mice and trackballs, I haven't seen a single custom build (not ploopy, not a few other trackballs or mice I've seen) that uses something other than microswitches, perhaps with a level arrangement, that gives travel in the range of a single mm.
What would be wrong with a mouse (or a trackball) that used mechanical keyboard switches for buttons?