Which really begs the question: why not have it open-source at that point? Obsidian isn't making money from things hidden in the code, but rather their Sync service.
Might as well open-source it (and perhaps get more people helping with the development), keep the Sync service, and stem competitor projects like these in the bud.
I suspect it's mostly about setting the expectation. They don't want to give up the control, they don't make it "free" (although it virtually is). Both are possible with open source but it would need a lot of explanation. Being closed makes it more natural.
Because then someone might fork it into a new product with their own sync service.
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"Open source" is not same as "source available".
"Source available": you can look at source code, maybe run a modified version internally.
"Open source": you can integrate it into your own software, republish, etc.