In many sports, there’s selection for the level one competes at. You won’t see Messi play in third division football, for example, or me being offered a job by a top team. Top athletes want the challenge/fame/money of competing with better players.
In other sports, there’s a rating system that effectively places people with competitors at their level (ELO in chess, tennis rankings)
Either would work in any sport, except for the fact that huge classes of people wouldn’t ever be represented at top level. Prime examples are women and people with disabilities.
In para-athletics, they try to solve that with an elaborate classification system (https://en.wikipedia.org/wiki/Para-athletics_classification)
Problem is that training can sometimes move you between categories. For example, https://en.wikipedia.org/wiki/T42_(classification) has “impaired muscle power or impaired range of movement” as criteria. Those can get better with training.
For women, it seems easier, but even ignoring transgenders, there’s various genetic rarities such as https://en.wikipedia.org/wiki/46,XX/46,XY that can put women at an advantage (https://en.wikipedia.org/wiki/Foekje_Dillema) that we may or may not choose to call unfair (we don’t think it’s unfair for men over six feet to compete in basketball, though, even though that’s largely genetic)