Why are SIGHASH flags signed as 4 bytes when only 1 byte is included in the transaction?
I assume it is simply the result of lazyness.

In the original client source code (and still today), the sighash type is represented as an int. The serialization framework by default serializes ints as 4 little-endian bytes.

I assume Bitcoin's creator did not bother converting it to a single byte before serializing.

If there is any other reason, I'm afraid you'll need to ask him.