The rise of big tech is basically a market response to decision-makers who acted like you observed. When you've got a bunch of executives who will drop millions of dollars on open-source software because they needed to check a box, that's a huge economic inefficiency waiting to be arbitraged away. What "big tech" founder/CEOs did was say "Well, I understand the technology, I can learn the domain knowledge more easily than these buffoons can learn the technology, how would I build a company from the ground up to compete directly with them and steal all their customers rather than playing the sales game?"

And with a suitably driven, intelligent, and business-savvy founder, it works [1]. The whole "software is eating the world" philosophy is based on the idea that it's better to go radically vertical and bring technology straight to the consumer rather than deal with the inefficiencies of enterprise sales.

[1] Kind of. The key here is "suitably driven, intelligent, and business savvy founder." Most technologists are not such - they not only don't know the domain, they don't care enough about it to learn. So there's been a bit of a pullback to traditional enterprise sales for SaaS businesses in the last couple years; many of the consumer markets that have not yet been revolutionized by big tech have pretty thorny barriers to entry that no technologist has yet found their way around.