The treat method is successful because you are rewarding the behavior that you want from your horse, with something that the horse finds rewarding (the treat). The more you reward a specific behavior, the chance of the horse offering that behavior increases, and you have more opportunities to reward it. Once the behavior is established and on cue, whether a verbal “stand” or a specific location/orientation to the mounting block, the reward can be random and intermittent.
You can vary vary what type of reward is given (treat, pat, praise); this is the ‘random’ part of the reward schedule. And you can also vary the frequency of when the reward is given; this is the ‘intermittent’ part of the reward schedule. Once the behavior is solid, it isn’t necessary to give a great reward each time, but many people do for mounting, since that is a moment when the horse must be still for the sake of the rider’s safety. Nothing wrong with rewarding that each and every time.
Every now and then it is helpful to give a ‘jackpot’ reward, much more/bigger than the horse may expect, especially if you are working on something the horse finds difficult, and the horse makes an exceptional effort. You, as the rider/handler, decide what behavior or response from the horse is so outstanding that you want to make a huge impression on the horse. Ending the work session, hopping off with praise and pats, and/or giving a handful of treats are things that could be considered as “jackpot” rewards to the horse.
Think of a slot machine in a casino. We don’t “win” every time the arm is pulled, but every now and then there may be a small payoff. And then, rarely, there might be a huge payoff. Humans tend to continue to play (offer the behavior of pulling the arm again) because the next play just might result in a huge payoff (jackpot). We don’t know which pull might reward us with a jackpot, so we tend to continue playing. The horse may not get a treat each time he stands for mounting, but he never knows which time might result in a reward, or which time might result in a huge jackpot reward for standing.