That's new, it didn't use to happen.
I don't think that's completely right in context.
A lot of the niche guitar players we know for their technical ability didn't get deals strictly because of the bands they were in, but because of their activity on the clinic and educational circuit that was popping up in the early 80's. Part of that was being a salesperson for the brand that payed much of the costs of that circuit.
Paul Gilbert got his initial Ibanez endorsement, not for his work in Racer X or Mr.Big, but his notoriety amongst the Shrapnel Records and GIT guys from doing exhibitions and clinics.
Joe Pass would basically break out into an Ibanez ad when playing the sit-in/trio rounds all over the world between sets.
When you see the format of a lot of YouTube Guitar Influencers it definitely harkens back to how clinics felt back in the 90's. You play some guitar, maybe not a fleshed out song, but snippets. You talk about gear. Play a few more snippets, talk some more shop, maybe discuss techniques or theory a bit. But it always comes back to selling the gear.