It's all about reach and how much product a company thinks that it can move via a personality. Someone likeable with 150K+ followers is going to make some good money in direct compensation (forget AdSense) saying good things about a shoe that they get paid to test. A ten-minute YouTube video calling the new Acme Footwear GoMonster X6000 Plus, "fast, fun, and a joy to run in!", sells shoes. Most importantly, it sells full price shoes to hobby joggers with jobs and disposable income that will buy 2-3 pairs each year plus a coordinating garment or two because they are going to run that marathon the Shoetube guy just ran - it looked like fun!
An elite with 30K subs posting something like, "Eating pizza in Budapest with another skinny famous runner!" or "Rise and grind, Flagstaff style!" isn't moving the needle. Sure, some high school kids bought the Dominator 6% Hyper Spike because their hero wears them but, elites are terrible at pitching the products that they use. Look at posts from just about any elite athlete and you see more direct promotion for local bakeries and their buddy's podcast than you do for their primary sponsor. What's Grant Fisher's go-to daily trainer? I have no idea.