'How come so many pilots get to the level of experience of an IR and still don't know how to fill in a logbook?'
Surprising, I know, but there are people out there who should know better. You ask them when you are low houred, wet behind the ears and naive. Whatever their reasons, you follow their instruction, fill your logbook incorrectly and - presto - come the reckoning with the CAA, you discover that they knew jack, took your money to fill PIC hours and the only comeback is to you and your wallet. The CAA are totally disinterested in educating the monkeys and it is up to you to rectify others' hubris/dishonesty.