why can't I download the manual for apple watch 7?

It is available via Apple Books (free), so you can then read it on your iPhone, iPad, or Mac

https://support.apple.com/guide/watch/welcome/watchos

There is also this short PDF, which you can download

https://manuals.info.apple.com/MANUALS/2000/MA2028/en_US/apple-watch-s7-03404633-info.pdf

Or you can download it as a pdf through a windows browser;