That's because publishers these days basically require you to have that magnitude of social media presence or they generally won't touch you. If they do, they will do next to nothing to help actually sell your book after it's printed. Very rarely will you see someone who hasn't built a platform already be given any sort of extra marketing or distribution for their work. You'll effectively give them 90% of the sale price for printing and possibly some limited distribution. Publishers used to be tastemakers and make picks and bets based on book merit, but now it's basically like they're just looking for things that would already succeed on their own and injecting themselves into the process.

They've basically figured out how to take half of their job and shove it off on the author while they still take their oversized cut. It's pretty egregious in my opinion.

I've seen this with all types of publishers, btw, from children's books to technical books. Heck, most technical publishers these days are mostly print on demand, so you're barely getting any unique product from the publisher at all.

The Martian is an outlier in its success, but it's far from the only instance of successful self-published sci-fi. Recent examples I've read include The Powers of Earth[1] by Travis Corcoran and Theft of Fire by Devon Eriksen. In both cases, the authors quit their day jobs (I think both worked in software) and are now full time writers.

1. https://en.wikipedia.org/wiki/The_Powers_of_the_Earth