Help with motivations on Player driven games

I hope someone can give me some help. I’ve been running a sandbox campaign for a few sessions now and i’m having the problem on how to explain to my players what are good character motivations.

They just "want to go on adventures" or "fight evil", but they don’t follow any rumour or thing it happens on the local area.

I would like to include things on the area about what is interesting for the player (or the character) but the PCs are very bland and its very hard to prep on that.

Can anyone give me some tips?

Cheers!