Why don't you therefore just wait until indexing is done?
I didn't know in advance why Siri can't access my photos (turns out it's because indexing isn't done) - at the beginning I thought there was some sort of other problem. Later I figured out (after going through privacy/permissions stuff) that it's too stupid to know why it can't see my photos.
Amateurish.
I'll answer your question though: because it doesn't show anywhere the progress of the indexing.
It's on the list there alright.