A (signed) 64 bit timestamp can store a date much larger than the age of the universe.
On many platforms this would be a micro-optimization, but in the context of Ethereum, we want to squeeze every drop of gas out, don't we?
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A storage slot is 256 bits, as are hashes, ETH amounts etc, so it's simpler to standardize on returning a 256-bit number.
However, if you need a timestamp in your contract you are under no obligation to store 256 bits: It's simple to cast it to a smaller size. This will often provide a gas saving if you're storing other small pieces of data at the same time.