perhaps some of them are really dense to not pay heed when corrected, but haven't met as many trainers. i would be inclined to blame those who organise certification-courses for them and compile/design the course-material. one trainer, a quite polite & humble chap from a rural background, i asked him from where was he pursuing his course, and he named K11 (Kaizzad's course). similar goes for Leena Mogre's school and perhaps many such fitness-industry businessfolks. i had later cross-checked some of the tips these people so enthusiastically distribute to customers, and found such 'legends' to have been done with long back by the better-informed pros and researchers. didn't really expect these trainers to go out of their line (some don't have the resources or/and inclination to find out more). they just keep prattling the same lines taught to them, and wouldn't care if anyone tells better or corrects them, as neither we have the credentials of an 'authority' that distributes certificates in exchange of their hard-earned money, and nor are we capable of affecting/enhancing their job. that's often the fate of 'phokat ka gyaan' (free advice) - it slips off ears and minds uninterested in listening & learning.
though yes, the 'pricks of the gym', i would mostly nominate for that honour from the gym-bandwagon the likes of 'mobile-lifters' (who work-out with their cellphones more than with the weights, or make their phones work-out more than themselves), the 'capt. upper bodies' carrying (invisible) logs under their arms, the singers & dancers (more interested in shuffling tracks on the gym's music-system, increasing the volume with their decreasing interest, singing, and dancing between sets), and the serious onlookers. they would sometimes gather around or intrude your space to see what blasphemous exercises are you doing, sneer & pass on some comments, esp. if they have managed to bloat up their bodies and you've just started out or returning from a long break, and then engage in their self-righteous work-out and ego-massage routines. perhaps the best remedy to this, as some fellow participants of this thread have realised, is to may be smile back, ignore them, keep continuing with your regimen, and then see their eyes & faces a few weeks/months later, when they start dispersing. a few honest ones among them do relent & repent.