It's easier for the rider if they don't have to work with all their horse's issues all at once.
You know what drives me bonkers, is when riders point fingers at the horse and it's all about the "horses issues"
Our horses reflect 100% of what we do in the saddle, or lack there of. Instead of blaming the horse and the "horses issues" why not stop and look at the rider.
As Ian Millar says "A good rider blames themselves, a poor rider blames their horse"
So if the horse is upside down, stiff and not flexable - look at the rider! You have to be correct, for the horse to be correct. You have to be put together and using yourself properly, in order for your horse to use themselves properly. Your out, your horse is out.
I lose respect for riders, when they blame their horses. As Reiner Klimke says "It isn't our horses job to understand our language, it is our job to understand theirs so that we can be as clear as we possibly can, so that they can answer us to the best of their abilities."