Part of it is security/privacy and providers avoiding liability. People constantly copy passwords, tokens, personal data, etc., so clipboard history is risky by default. Apple probably does not want to expose a rich API here and then be responsible for securing that surface forever.

So macOS does not really give you a clean "this app copied this semantic object" API. Clipboard-history apps generally poll NSPasteboard.changeCount, which already makes provenance fuzzy, since you can observe that the pasteboard changed, but not reliably know the source app.

Pasting is fuzzy too. You know what representations were available, but not what the destination app actually accepted, because that decision happens inside the app and is generally opaque for the OS. So what even is history? Is it the raw object, the fallback text, the richest representation, the thing you intended to paste, or the thing the target app consumed? And even if you define history as "the observed events", polling can also miss states. And once you add transforms (like I want to), you basically have to define your own history model. A coherent OS clipboard-history API probably will never happen without big effort and liability policy changes from providers.