It has bugger all to do with money. Nits don't care what income the head has!
My two go to a private school and dd has had them 4 times. ds never has, possibly due to the shorter hair, possibly because boys tend not to do as much 'heads together' play with their mates!
Some people either
a Can't be bothered
or b are too disorganised to do the combing properly
If you do it properly the nots get smaller and smaller. If you suddenly get a big adult on in the middle it means your child has been re-infected and then you have to start all over again. In the past this has happened over and over again and it has taken a holiday to shift the little buggers for once and for all.