School council representatives are voted for by their classmates in the primary schools I've been in. Anyone can run, everyone votes.
Play leaders, everyone who volunteers gets a slot IF they commit to the weekly schedule. They'll lose their role if they don't show up because they decided to do something else.
School plays, well, yes often the same (frankly, talented) few get the main roles year after year. But you know what? They gave out a few choice roles to overlooked children last year because there was grumbling... and you know what? They were awful. Couldn't carry a tune and they were truly cringeworthy for everyone sitting through their singing. And we couldn't hear their lines. Just awful. Primary school productions are bad enough without adding that into the mix.
Sporting events? Yes, the kids who excel at particular sports tend to get chosen. They work hard for it, and they should get the chance. For sports that don't require as much 'coordination', others are chosen to give them the chance.
Academic events? Yes the kids who excel at particular subjects tend to get chosen. They work hard for it and they should get the chance. For academic events that allow more than one team, others are added to give them the chance to represent the school.
Yes, it can grate when a few seem to excel, but such is life. And I do believe most schools do try to give them opportunities to shine in other areas, even if you don't see it.