Max weight of child, for rear bike seats - why is it always (seemingly) 22kg?
While Criggie's answer explains the reasoning behind the limit, the reason why the official limit is almost always exactly 22kg is simple: that's the standard.

The 22kg limit comes from the European standard EN 14344:2004: 


  This document specifies requirements for child seats for cycles, which
  are intended to be mounted on pedal cycles and electrically power
  assisted bicycles, in order to transport children with a weight from 9
  kg up to 22 kg (approximately 9 months up to 5 years) and who are
  capable of sitting unaided. NOTE Some European countries have special
  legislation for child seats for cycles. Compliance with this document
  may not meet this legislation.


So that's why it's always 22kg, and not 23kg or "approximately 22kg".