The reasoning is quite simple and obvious: We have long-standing statutory and case law that defines specifically what is or isn't eligible for copyright protection, or patents, or trademarks. Your gut feelings about whether your API deserves protection because of how much effort you put into it are completely irrelevant to that legal framework.
There was a time when software didn’t have copyright protection. Somebody had to make the first move.
And all of the above is about the legal, technical questions about what the law is. The public policy question of what the law should be is a different matter entirely, and opinions about that generally should be addressed to Congress, not the Supreme Court.
I think you're mixing up different types of intellectual property:
- Trademarks are intended to identify a source. Trademarks cannot be functional, they have to be descriptive.
- Copyright must express a creative idea. That idea can be artistic, or functional. Computer code and APIs can be copyrighted (as they most often are), but they can also be patented.
- Patents cover inventions with some utility. One can definitely patent a piece of art if it provides some utility.
It's not uncommon for companies to throw all three at whatever they are cooking up.