It isn't everyone, but a lot of people are starting with 1080s or threadrippers or whatnot. I'm building my first computer, and it's still expensive, a little below $900. It just seems strange.
Why wouldn't you? You don't need training wheels to build a PC.
because they have the money for it and also simply cause they can.
Or this. Probably won't be the case for most people though.
A lot of beginners in this site are newbies to PC building, including me. I had no clue what made each part different and generally thought the more expensive a part was the better it would be. I joined this site when the Ryzen Revolution so was just beginning so videos on that also helped me see that Intel is not better at everything and have their own flaws, just like any company.
I think that many people who are building their first PC are coming from consoles or older PC's. They are probably looking for the total PC experience. a $900 pc is not that expensive in the custom PC world. Thats not to say it isn't expensive (heck, I just bought a $750 laptop for school and light gaming, its definitely an expense!). A 1080 is high-er end these days, but really a lot of people are probably looking for decent future proofing. they may not want to have to upgrade in 3 years or so. I personally will probably be spending $100,000+ on computer stuff over the course of my life (no problems there!). I guess it comes down to what you want in a PC. some people also build these for heavy workstation use and not gaming.
I went big when i built my first PC for several reasons.
1. An investment in case i'd need to sell it for quick cash
2. Full gaming experience at the highest level.
3. Spending the money now on the PC i wanted instead of settling for something else and end up spending more.
4. I've made the mistake of "settling" for a pre-built i wasn't happy with and ended up investing more overall that i would have if i waited.
5. Future proofing so i dont have to do upgrades for a while.
If you have the money go for it, but you always have to be practical. i spent 3k on my current rig and i RARELY game on it. I have more time spent on twitch than actual gaming.
If you have to money to build a more powerful system then you do it. Why not go 4k ultra high if you can? If you don't have the money then you build more of a budget PC.