What reason(s) would require these projects to have a server, versus deploying a static frontend to S3/Netlify? My understanding is that the "backend" is the blockchain, and transactions are sent client side with
There's not fixed rule, but usually part of a dapp may work on-chain and part off-chain. While a static frontend with some js can satisfy the former, the latter may require additional resources (e.g. bandwith, database, mail services, ...).
...btw many projects are still hosting on cheap VPS instead of the cloud ;)