A question from a naive beginner in the UK.

The following questions are asked from the perspective that, other than aircraft differences, inso far as I can see there are no practical/competence differences between pilot skills in flying GA aircraft under the same VFR restrictions so...
  1. For pilot licensing what practical differences are there between an SSDR, a 2 seat microlight and higher weight class aircraft in the UK?
  2. Shouldn’t those who are qualified to pilot SSDRs and 2-seat microlights also be qualified to fly higher category aircraft under VFR restrictions with their NPPL licences? If not, why not?

So far...I can't see any logical or safety/risk-associated reasons for the differentiations.