There's a couple of potential reason(s):

• name + address farming. scammers deliberately accept offers just they can generate pre-shipment labels that have your name, address (maybe attached phone #'s?). they can then sell confirmed name + address on the dark web to potential fraudsters who may have more nefarious uses for it
• they are actually trolls that get off on doing stuff like this. yes there really are people that bored in this world to do all this
• some scammers are new to the scamming game and think they will have access to those funds right away. only to fight out that it's on hold. so they give up and just don't ship out an item they don't have

One of many reasons I never recommend users to buy/sell via shipping option thru Offerup. Use Mercari or eBay for shipping.