For example you can make yourself steadily stronger by gradually increasing the amount you're squatting each week or by running longer distances. You don't have this level of control when playing competitive sports.

That said I agree with the social elements. Sports are great fun and joining a team sport would be worth it for the camaraderie alone.

One characteristic of aerobic athletics is that most individuals peak in their mid 20s to late 30s, after which raw capabilities typically decline (albeit gradually, less than 1% per year). There's some exception for endurance activities, in which maturity, pacing, training, and nutrition can provide gains until later years (highly competitive 40, 50, and 60-something ultramarathoners aren't uncommon). And for activities involving fine motor control or finesse, skills may be developed over a lifetime, though injuries can also accumulate: gymnasts, dancers, and football players often peak out young.

For strength sports, progress is cumulative over years, meaning peak strength may not occur until the mid 30s or early 40s.

In competitive sports, you're looking at a mix of skills and abilities. Individual progress is harder to measure as it's team results that are most evident, not that stats aren't available (how meaningful they are is another matter). High-contact sports tend to be games of youth, but this isn't always the case.

And for anyone, coming to an activity fresh means a lot of latent potential to unleash: you'll likely improve dramatically over the first few years of participation.

Stretching happens after if ever, not before :)

From what I know: Stretching after doing a workout is bad, because it tenses muscles even more, stopping the flow of blood in the muscle and thus, making muscles more sore. But maybe that opinion is just the current trend.

on phone, but sources are a quick search away.