Why does the Speaker of the House never preside over the House?

I've been watching C-SPAN a lot lately, mostly the House because I've always been interested in the Speaker's role because of how much power they actually have and the Senate's quorum calls are unbearable honestly. I was wondering if anyone here actually knew why the Speaker never presides over the House? I've seen Nancy Pelosi do it once and that was to swear in Judy Chu. Why is this that the Speaker always designates someone else as the Speaker Pro Tempore instead of herself/himself?