There is more to S3 than just serving static objects off of the local drive. Maintaining the integrity of the data and ensuring that the data sent out is consistent is not a trivial task. A constantly changing map with 2 trillion keys is a hard problem on its own. Also, serving 1000 1MiB objects per second is not the same as serving 1000 1KiB objects per second so it's hard to say how many resources just the serving portion consumes.
I was assuming that the number of requests referred to read requests, and I guess that the system is designed in a way that makes read requests very cheap, maybe even cheaper than reading a file on a usual filesystem, at least for hot data.
Just because it's huge and complex doesn't mean it's slow and that requests are expensive.
