Because over the years there have been numerous issues with them.
Without pointing fingers at individual schools
Poor quality of training
Sharp buisness practises
Not being all together truthfull in advertising
Unpleasant characters involved running them.
Sausage factory production.
Customer service very poor.
To be honest though the unpleasant characters is the main reason for most of the posts. With most of them there are literially hundreds of unsatisfied customers that have a point to be made.
Some folk can have a good experence out there learning and others have a terrible time.
Personally I did the FL PPL thing and its wasn't until I did the commercial and FI stuff I realised how poor the training was. After the FI I realised you couldn't really do a PPL test in a hour. Normally it includes a PFL, a nav leg with a divert before you get to the first turning point. And that divert involved turning right to run up a river and count 2 bridges with the first bridge your half way point. The RT test isn't someone banging on the wall everytime you make a mistake and you just having a go again until you get the call right.
Just to prove that this isn't across the board if you go back far enough there was a school called Brittannia run by 2 lads that never got anything bad said about them on PPrune. Both of them moved on and the school shut which was unfortunate. Everything wasn't perfect by a long shot but everyone was happy and it was a good laugh there; even if the school itself was a bit ruff and ready.