The distinction is a legal one: intellectual property can not be re-used without permission of the rights holder, be it a patent or a chunk of source code.
And you can bet that SpaceX using physics knowledge and cumulative rocket engineering knowledge are very careful to either license the tech they use or be very explicit about documenting their own.
That you can't see the difference is entirely on you, going 'against the flow' of society sometimes leads to change but more often it simply results in friction and a lack of comprehension.
Keep in mind that open source is based on copyright law, and without copyright law the protections that open source offers would be gone.
To give an extreme example: if you had a chunk of software that was constructed in such a way that it would spit out a complete copy of 'the Gimp' without the license file if you started to write an image processing program that would be a very clear case of copyright violation.
If you then start breaking the Gimp down into smaller and smaller re-usable fractions at some point you might be able to argue that such a generic and oft used snippet should be free of copyright. But that only works as long as you then don't string together a whole pile of pieces that you each copied somewhere else, the whole idea is that your creation is an original one.
Medical research (which quite often leads to patents, which I don't believe should be possible, especially if that research was publicly funded) and physics knowledge are of a different kind than copyrighted program code. The latter would be better compared to universally present language constructs and constraints, such as 'memory management', 'data manipulation' etc. Once you make those explicit in an implementation copyright applies.
Or, to make another analogy: it's like comparing the skill of writing to the product of that skill. The skill isn't protected, but the output of the act of writing is.