The USPS is like this because of the persistent belief that it's not enough for government entities (think USPS, Amtrak, etc) to provide a good service for the citizens - they must also (try to) turn a profit.

If we as a society considered it acceptable for the USPS to spend money to ensure everyone in the US had mail access without selling out to corporations to turn a profit, they wouldn't need to have products like EDDM blasting spam to entire zip codes.