That rarely happens because most Instagrammers invest hundreds of hours of grinding work to get their followers (post frequency, posting at a certain times and days, deleting non-performing posts, obsessed with making it to the Explore page, etc). I guess that less than 1% of users with >10k followers got there "naturally".

... or just buy them like that guide that was on front-page

Most of my friends who are good at skateboarding or surfing have 2-15k followers, regardless of whether or not they're sponsored.

They do almost no personal branding, they're just well-known in their area. I imagine it's the same for most sports with a lot of participants, save those with a high financial barrier to entry (e.g. racing cars).