Ask HN: Is it me, or are more and more YC companies open source?

Exactly. Also, nowadays being "open source" is one of the only ways to gain technical-minded customers. The level of skepticism against closed-source things is high (and for good reasons, between vendor lock-in, guaranteed future enshittification which means you want a fork option, etc). Personally, I won't even consider adopting something that isn't open source anymore unless it's a product that I (or my company) really needs, and even then I take steps to avoid hard dependence.

And it gives the feeling anytime one can self/diy. Like kubernetes. Most can't aa it is too complex(and unnecessary) but gives a safety feeling.

"trending" suse and red hat have been doing it for decades. Canonical too, among many others

You were downvoted but there is some truth to this.