Like every other industry, you first do it yourself, make a name for yourself, then you'll start getting sponsors. and most modders do modding because they enjoy modding. Not all mechanics are race car drivers. Not all race car drivers are mechanics.
I think it's complicated. We need to tackle one thing before we get to the why though. Firstly, almost nothing is free. Sponsorships are a contract between builders and a company to provide marketing. If you haven't been through it a couple of times, it sounds like a dream, parts just suddenly appear on your doorstep and you get to build your dream system. Poof! Wish granted. But I'll walk you through my Twelve-80 build for an example. First things first, you sign a contract naming what is expected out of you, ie posting build logs on a certain number of forums, regular updates, time constraints, etc in return for a set amount of hardware or payment. Wait, actually before that, you spend at least a couple of years dumping your own money into builds to prove you have the chops, then you sign a contract. You get a design approved, then you get some parts. For Twelve-80 it was everything but the case and PSU. Let's say it was $2500 worth of parts, at least in that ballpark, I never added it up. And that was my payment, the parts, $2500 retail. Then you go buy all the things you didn't get, in my case, the PSU and about $200-300 materials for the case(you don't rattle can this stuff). Then you spend 80+ hours using your own tools and workspace to craft this build and however much time it takes to do the forum updates, pictures, etc, etc. So at this point I've made $2500 in parts, spent $500 of my own money, and put in 2 weeks of work, so I've made about $2k in two weeks to provide MSI, Bitspower, and HyperX marketing at a targeted demographic(modders) and marketing material for however long they want to use it(pics, etc.). Extrapolated over a year, that's about $50k or around the avg US salary. If I can pull down a $50k salary for doing something I really enjoy, sign me up. But realize, at this point, you're making the average US salary, and you're not getting anything free because you've put time and money into it. You have been paid for a job. Decent pay, but a job nonetheless. And here's the rub. Now if I would have built that system for $2k cash money, finished it and shipped it off, I'd have $2k in my pocket to pay my bills and continue to eat. But since I did it in parts, I have $2k in parts, which my utility company doesn't care about and I can't eat the parts, and oh yeah, since they've been used in a build, they're now worth about 3/4 retail or less. So in the end, you end up with a computer, which is great if you need one, or $1500 if you sell the parts for cash, not horrible for a hobby on the side. My point being, none of it is free. Sorry to unpack all that on you. I'm not trying to be mean, just informative. Some people think you just call up NVIDIA and say, "hey, I need a card, I'll instagram a pic of it," and magically you get a 2080Ti. If it were only true. As for the why, personally, I like building cool and pretty things. Sponsorships help me to that end, but even without I'd still build cases. It's all kind of a mix. Some of it is glory, pride and recognition. There's some competition and conceit. A bit of showing off and proving I can do something. My personal mission right now is to find a replacement for my favorite case that I retired. So on that front, it's very much for my use. I built Leaf Me Alone for my girlfriend so she'd have a nice computer to use. And some of it's just for marketing. If someone wants to pay me to do something I enjoy, I'm game.
Oh, and also, EGO. Of the people I grew up with, I think I'm the only one who's been internationally recognized for what they do. It's like the Chris Evans going to his HS reunion, just on a much, much smaller, niche scale. And with everyone saying, "I didn't know that was a thing."
More like putting cocaine in front of an addict, but you're tracking. But it's less permission and more of paying for ad space. Think Nike giving a star athlete a pair of shoes.
Speaking for myself, I found that really informative. Interesting to hear how it can all work in reality.
