[brainwad]

People say the universe is "infinite" because spacetime's curvature is, as far as we can tell, flat, and so it should continue in all directions without ever wrapping back on itself (unlike, say, the Earth, which has spherical curvature).

But practically it's finite because we are only in causal contact with things up to 13.7b ly from us, and given space appears to be expanding at an accelerating rate, we probably will never get into causal contact with (almost all of) the part of the infinite universe outside of our light cone, even though things ought to exist over the "horizon". So only a tiny infinitesimal sliver of the infinite universe is knowable by us.