Why are clickable pathname links not readable on a PC from a Mac?

#1: define 'clickable pathname link'.

Where are you seeing/using this link?

Ultimately, the answer probably lies in the fact that Macs and Windows systems use completely different path formats for files at a local level.

Early MacOS systems used colon-delimited paths and full paths include the name of the disk. In addition, MacOS paths allow almost any characters in file names (except :)

Macintosh HD:Users:username:Documents:Family & Friends:Malmö vacation:Pictures:Some.jpeg

Windows systems use backslash-delimited paths and early Windows systems were based on DOS and its infamous 8.3 filenames and didn't allow spaces or diacriticals:

C:\\Users\username\document\family_f\malmo_va\pictures\some.jpg

Both have evolved over time. MacOS now prefers Unix-style, slash (but not backslash)-delimited paths:

Macintosh HD/Users/username/Documents/Family & Friends/Malmö vacation/Pictures/Some.jpg

and windows no longer enforces 8 character filenames and allows spaces and diacriticals, but it still uses drive letter references and backslashes:

C:\\Users\username\Documents\Family & Friends\Malmö Vacation\Pictures\some.jpg

However, there's still no clear, direct correlation, especially at the disk level. There are some levels of automation that will replace backslashes with forward slashes (and vice versa), but it's still not the whole story.

So it's a long, messy history that stems from decisions made in the early days of computing that we're still paying for. :)