Athletes are told what to take on during a match and then handed it for the first time on match day.
Eating while you compete is a skill that needs reps
The gel goes in at forty minutes. That is the plan, that is what the sheet says, and the first time the athlete has ever swallowed one while running is in a race they have trained six months for. Then it comes back up and everybody blames the product.
Taking on food and fluid mid competition is a motor skill wrapped around a tolerance problem, and both halves need practice. The motor skill is the awkward physical business of opening something with cold hands, drinking from a bottle without losing rhythm, doing all of it while somebody is trying to beat you. The tolerance is the gut, which behaves very differently under match intensity than it does on the sofa, and which adapts to what it is repeatedly asked to handle.
Neither adapts to a briefing. They adapt to reps. So put the reps in training, at the intensity of competition, using the exact product and the exact bottle that will be there on the day.
Team sports have the same problem in a less obvious form. A cricketer batting through a long innings, a footballer with a cooling break, a hockey player rolling on and off. There is a right way to use those windows and it can be rehearsed in a practice match at no cost, yet almost nobody scripts it. Instead the athlete improvises at the moment when improvisation is most expensive.
The other reason to rehearse is that it exposes the plan to reality. Half the fuelling schedules I have seen are physically impossible to execute in the game they were written for, because whoever wrote them was not standing on the pitch. Try it once in a practice match and the impossible bits reveal themselves immediately, which is the whole point of a rehearsal.
Coaches skip this because it feels like a small thing and training time is short. It is a small thing that decides the end of long events, and it takes about four minutes of a session to include, mostly because you are not adding a drill, you are just refusing to pause the drill you already had.
If the first time an athlete does something is on the day that matters, you did not coach it. You gambled on it.

