Short answer: Filmic device. Of course Roth could have had Michael (and the bodyguard--formidable as he looked, he was only one man) whacked the moment he stepped off the plane in Cuba. Or he could have had his pals in the Cuban government arrange to grab the money (at Customs) as soon as Fredo stepped off the plane. But that would have deprived us of the supremely satisfying cat-and-mouse game he and Michael played out, to our everlasting pleasure.
Longer answer: Roth needed to have Michael hand over the money to provide him with an alibi and longer-term insurance once Michael was whacked. As I said in another thread: the birthday party scene was designed to provide Roth with an alibi in case Michael's successors started asking questions once he turned up dead (or was missing): "Hey, I loved the kid like my own son. I was gonna turn over everything to him. Just ask those fat thugs who were at my birthday party..."
Also, simply overwhelming Michael and/or having his money confiscated would be equally damning for Roth. I believe he intended for Batista's secret police (SIM) to attack the military car that was going to take Michael back to his hotel after the party, and kill everyone inside the car--making it look like the rebels did it, not knowing that a gringo was in the car with Batista's soldiers. Then Roth could have said, "Gosh, I'm really sorry about this...looks like poor Michael's a victim of the rebels."