I think it all comes back to family-as-unit and the shower being less a party and more about setting up for a new baby.

If the close relative throws the shower that's a little too close to 'give us stuff' but if a friend does it, that's more 'hey, Laura's gonna need stuff for the baby and we should chip in and help her and Tom out' and so on.

An old cultural taboo that you don't ask for stuff but being given it is okay.