Why do you need blockchain for smart contracts?

Sorry for this crazy basic question, but it's one that is bugging me.

The case for smart contracts has been around for decades. Nick Szabo's essay is well-known. He did not think about blockchain, only that one day interconnectedness will enable effective smart contracting with real-world applications.

I understand the difference between a blockchain and a centralised server.

I, for one, do not believe that the "idealistic" side of blockchain is enough to ensure its adoption. That idealistic side can usually be summed up as "decentralised is better" and I think it's, at the very least, debatable. But I am also not sure real businesses really care.

I understand the use case of smart contracts, smart contracting and machine contracting. I do believe there are fantastic applications for those.

But please can anyone give me a compelling argument for why you need the blockchain for this and not a very reliable centralised cloud provider?