> These platforms are for casuals.

At least in road and trail running, this doesn't line up with my observation. There are plenty of elite athletes, at least in the US, who are posting their training on Strava (Jim Walmsley, CJ Albertson, Molly Seidel, etc.) At the fast amateur level (e.g. those chasing the Olympic Trials Qualifier time), Strava adoption is pretty ubiquitous.

Out of the the athletes who are active on social media in the first place, those who don't post on Strava are usually doing it either for privacy reasons, or because they do not want to share their training plans for their competitors to copy.