Believe it or not, it's a kind of altruism. You've benefitted for years from these networks. You get to know the people involved. And now you have the skills to contribute something back to the community.
(That and, as a sibling said, being taken seriously despite your age)
I have no personal experience but from my observations I see it like this:
Number one you are a part of something big, even small groups can have massive impacts from their leaks/releases. It's no fluke that the demographics for scene members often favors younger people, I know that for the internet in general, when I was younger, it was a magic place where age didn't matter, only merits. I would assume the scene is no different. It doesn't matter how much you make, and your race, religion, age are not important. You are judged by what you can DO not who you ARE. That alone is a VERY powerful motivator.
Secondly I think the scene is very appealing to people who have a tech inclination plus a love/hobby for something else (Software, movies. music, etc). The scene allows them to interact with people who share the same interests and gives them access (through top sites or similar) to more content that they love.
Lastly sharing. When I like/love something there is a part of me that wants to share it with everyone else. When that something is funny story then all is well and good but when it's a TV show/movie/album/etc it's a little harder to share. Sure you can share a link to Amazon/Netflix/etc but unless your friend already pays for access then you share isn't "free". It's free to you but not to the person you are sharing. Piracy makes it easier to share because you don't let those pesky laws get in the way. Even so sharing is not 100% free, you can tell someone about a piece of content but they still have to go find it, download it, and then they can watch it. Topsites are like your friends opening up all of their content so that you can sample it on your own or go grab something on their recommendation. There is still a little friction to the sharing (a direct link to watch would be ideal) but it's about as close as you can get.
That's just my 2 cents.
To answer directly to: What motivates someone to risk so much and invest so much time and money into participating in the scene today?
I, for example, love messing around with web dev. Because of this I have tried out (and continue to pay for) a number of online hosting platforms (both dedicated, shared, and your AWS/DO/RS/Google clouds) either to try them out or because I am long-term hosting things on them. This ends up being a fair amount of money when all is said and done but it's my hobby, it's what I love and it benefits me at work. Now other than the "benefits me at work" bit I don't see how my hobby is any different from file sharers (legality aside). Even then I'm sure you can learn skills participating in a release group that can benefit you in your professional life (server management, release planning, etc).
Keep in mind that the majority of the torrents are actually scene leaks (never intended for public distribution). The scene is now a well known thing, but years ago it was a very exclusive club.
Certainly not the majority. Scene releases still feature, but they are no longer consistently the first to market, and they are almost universally derided for prioritising speed over quality.
Scene rules are anachronistic and slow to change. The MP3 scene stuck to -V2 VBR for far too long, for example; the change to -V0 VBR in 2011 came as the sharing of FLACs was already ubiquitous outside the scene, and it wasn't until 2012 that the FLAC scene was established. Even now CUE and LOG files are not required from a CD source, which is insane.
for me, it was about getting anything and everything the moment it hit the sites. It was just fun knowing I had access to it faster than most others. Most of the time spent in the irc channel was just bullshitting about what was going on in front of you.
