It's economics. Customer service: giving the customer what they want. If they come to you wanting to improve their competition results, that's what they expect to work on - horse performance in competition. They're so convinced they know it all, it doesn't even occur to them that the issue could be them. Or the gaping holes in their knowledge that they have no idea are there.
If you tell them how to improve the horse, preferably with instant results, you'll have a repeat customer. If you tell them the issues with the horse are caused by their poor standard of riding, they'll get offended and you'll never see them again. They'll also tell all their friends how shit an instructor you are, that you couldn't improve the horse and they've been riding for 5 years so of course they can ride and don't need lessons, it's just the instructor doesn't know what they're talking about.
Some training courses these days aren't actually in instructing anyway, they're coaching qualifications. People don't want to put the time and effort in to learn a skill, they just want to get to whatever they want to be doing as quickly as possible. They don't care how they get there.
The parents buy the horse because livery is cheaper than riding lessons, they have no understanding about all the other costs and don't really want to pay for them. Teenager doesn't want horse sold so keeps quiet about anything needed. Adults looking down on them doesn't bother them one bit, but they can't bear to not fit in with friends, so any money goes on new numnahs etc while the horse doesn't get the vet because it's "not that lame and it's only been a few weeks anyway".
It's not just teenagers either. A middle aged woman skipped a shoeing cycle because she was broke and "needed" the money for spends on an upcoming holiday. All her stable pals justifying it because "she's had a hard time lately" and "needs a break away" etc. Someone else ranted at the vet because they wouldn't provide the life saving medication needed for free because the owner couldn't afford to pay for it. As if the vet is a public service and somehow owes them. It's all me me me these days, the animal is secondary at best. People are becoming increasingly selfish.
As for "we all make mistakes" - you should make them during training, not once you have full responsibility for an animal that's relying on you to meet its needs. Another living being suffering needlessly because you "made a mistake" isn't acceptable. Except unfortunately, to many people it is. They believe their right to make a mistake trumps their pet's right to a decent standard of care.