Sirtak

They either have a dedicated stream PC that only does the stream from a capture card and no gaming at all, or they do both gaming and streaming on the same PC because it has enough horsepower (and NVENC encoder is "light" on resources as it uses the Nvidia GPU rather than the CPU, albeit at lower quality).
CPU encoding always has the best quality, so you need a beefy one.

because they've got money to blow, or they stream 4K 60fps.

You're missing the product sponsorship deal that gives you those parts for free in exchange for advertising them on your Twitch profile.
 
The 2700 (8c/16t) and 8700k (6c/12t) have more cores and threads than a 2400G (4c/8t), as well as higher clock speeds, and will perform better in the encoding tasks of CPU encoded streaming. The professional streamers will most likely be running a much more intensive stream, with 1440p 60fps or even 4k 60fps with a much better quality (slow) encoding preset that require the extra processing power. This won't be necessary for your use case.

Your sense of entitlement is off the charts.