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> Why should it be impossible for a third-party package to just create /opt?

It is not impossible, obviously, but /opt and such should come from the operating system's vendor, and the vendor should be the only one to decide which filesystem permissions to provide: 0755? root:bin or root:sys? root:root? bin:bin? The vendor should decide that, since a vendor is supposed to know their operating system best.

Third parties might not agree, or even decide correctly for that operating system.

This is system engineering and architecture, something which beside operating system vendors, software vendors do not have a clue about in the slightest.