How would they have taken it with them?

They did bring gold with them, and it did sell for a lot.

IIRC they had a blast "stealing" the gold and running off thinking they were doing pirate stuff for once, when the skypieans were actually running after them trying to give them more.

Robin enjoys watching the antics of the crew so I suspect she didn't want to ruin theirs, or her own, fun.

She thought it was funny, its just how she is (and why I love her)