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.
In the HN bubble maybe. The majority of people in tech do not care at all. For every one of us, there are 10 of them using SQLServer/Oracle/Windows/... without the slightest clue about how the licensing works.
A lot of these startups have founders from inside that bubble, which means their initial sales network and the market they know is that bubble. Which probably explains why they see it as a good idea.
I would have said they also have procurement departments that did have a clue so one wasn't needed, but now it's more likely procurement has been outsourced to Accenture who do it for highest fee, paying for the convincing PPTs and golf, and lowest Accenture branded service center cost, paying $900/month fully loaded average across the team - the struggling 'knowledge workers' that know neither your company nor its specific needs.
Most do SaaS or dynamodb or lambda type app engine. Once it grows significantly then administration will need oracle etc. But hey these can be done by external contractors