Speaking solely for myself, but gesturing in the general direction of my peer set: there exist some software entrepreneurs who have substantial operational expertise and have made some money, but who do not feel like they'd add much value in the unicorn hunt, partly due to access reasons and partly due to different skills. There's a distinct body of practice between e.g. the things a YC partner would tell you to do about fundraising, the things a growth stage VC fund would tell you to do about organization building, and the things a self-funded software founder would tell you to do about e.g. testing AdWords copy. There is a certain degree of overlap, in the same sense that all of them overlap someone whose business model is buy-and-hold apartment buildings in Chicago, but there is a reason none of them buy-and-hold apartment buildings in Chicago.
By comparison, I feel reasonably confident that "six figure SaaS business trying to get to seven" is somewhere I understand pretty thoroughly, would almost always enjoy a conversation with that founder, and (napkin math) looks plausibly mutually interesting.
Why now and not five years ago? It feels like a generation-of-business thing: the reason everybody who made money at Google / Facebook didn't invest five years before they started investing was because they hadn't made money at Google / Facebook. Did the opportunity exist five years ago? Probably, as evidenced by the successes from that generation. Is the opportunity better now? Plausibly yes, as demonstrated by all of the people who were successful writing down what they did, doing technology transfer on that to new operators (often for free), the ecosystem radically improving (inputs, platforms, employees, exit options all got a bunch better), etc.