The joists and flooring together function as a structural component.
It's important that they're well coupled to each other, i.e. where
every board crosses every joist.

Glue can produce strong joints where the joint is clamped up firmly.
"Glue bridges" as would inevitably happen with the variable gaps
between joists and boards, are not strong.

I screwed & glued legs on a table 20 years ago & they are still solid.

Exactly, tightly mechanically coupled joint + glue = strong joint