It's probably because procedures in different countries don't require full readbacks, however in the UK, there are a whole load of things that require a MANDATORY readback:
Level instructions
Heading instructions
Speed instructions
Airways / routeing instructions
Runway in use
Clearance to land / take off
Clearance to enter / cross or hold short of an active runway
SSR codes
Pressure settings
VDF information
Frequency changes
Types of radar service
I think that's just about all of them (if my memory serves me correctly, I would have checked in my MATS pt1 but i don't have one anymore
)
My main problem at my unit when working radar, is US crews not reading back the type of radar service they are receiving RAS/RCS
Or Crews not reading back the frequency change, just saying bye, then 2 minutes later coming back asking what the frequncy was after they've levelled of at SID altitude