It's not simply that developers expect to get their tools for free.
So many developers have seen the rug-pulls and exploitation of non-free tools. Build on Oracle and your company will need to hire more lawyers than developers. Even in less-exploitive situations, we've seen a lot of situations where things become many times more expensive. Google AppEngine moved from charging based on usage to charging based on instance hours and some people saw their bills go up 10x. We saw the Unity price increase which proposed a runtime-install fee. We don't want to build off an ecosystem where we have no idea what the pricing will be going forward. We don't live in a world where we can just remain on an old version via a perpetual license. Security vulnerabilities will require upgrading at whatever price a vendor sets for the new version. Incompatibilities with changing environments (like iOS/Android upgrades) will mean having to pay for upgrades at whatever the new price is.
We've seen so many proprietary dead-ends where we invest a lot of time and money into a platform and then poof it's gone. You don't want to have 10 devs spend a couple years building with a tool that just disappears on you. Something small like Skip could easily run out of funding. This gives you a chicken-and-egg problem: you can't be proprietary unless you're huge, but you basically can't get huge at this point unless you're open source because no one will choose you. Skip was ejectable. It was generating Kotlin so you could just start developing two separate codebases in the future, but if you want a cross-platform toolkit and you're worried about a dead-end, you're just going to choose Flutter or React Native or something.
We also don't want a situation where devs are waiting on a vendor. With open source, I can go in and fix something at my company and put in a PR. Even if the PR doesn't get accepted for a while, we aren't stuck.
And it's not just developers. If I'm working at a company and I want to use a paid tool, I'm going to need to get approval for that which can just be a pain. Higher ups are going to want to know that we aren't going to get a rug-pull in the future. Skip was $1,000/year per developer, but that could change in the following year. Companies have gotten rich by offering you a good deal, locking you into their ecosystem, and then raising prices. Higher ups are going to want answers that don't really exist.
Finally, it's hard to know whether something is any good without putting a decent amount of time into it. We often learn things because they're free toys we can play with. I make something fun in my spare time with a free tool and I've learned something new. But I don't want to do that with something proprietary where I might have to deal with licensing. Yes, sometimes there's exceptions for non-commercial use, but sometimes the line is blurry on that - what if I have a tip jar. We don't want to deal with that.
A development kit like Skip isn't a hammer. A hammer will continue to be a hammer even if the company goes out of business. When we're choosing tools, we're not just making a bet on what it is, but also what it will be in the future. If it's going to become abandoned in the future, it'll be a lot less useful. When you're comparing tools at a hardware store, you might not make the best choice, but you aren't going to find out 18 months later that your hammer is incompatible with all nails going forward. You're also generally only out the price of the hammer, not out the price of the hammer plus 18 months worth of work that you need to redo.