As someone else pointed out, the USPS has a monopoly on non-urgent mail.
As for spam, you must be fortunate that you have not managed to wind up on spammers' lists. If you donate to charity that's an easy way to get on the lists, and some of the spam is just unnecessary garbage sent by business I use b/c sending it to everyone (via the USPS) is cheaper than implementing an opt-out policy.
I forget the URL, but I opted out of junk mail years ago via the Direct Marketers Association web page. I think that's the name. After a couple of months, my dead tree spam fell to a trickle.
It's not perfect. I still get local grocery and retail circulars that I could do without. But at least the daily credit card offers stopped, which cut down the volume dramatically.
If someone would drop a bomb on Geico's direct marketing arm, I'd be a happy camper.
Article I, section 8, Clause 7 of the United States Constitution grants Congress the power to establish post offices and post roads, which has been interpreted as a de facto Congressional monopoly over the delivery of mail. Accordingly, no other system for delivering mail – public or private – can be established, absent Congress's consent. Congress has delegated to the Postal Service the power to decide whether others may compete with it, and the Postal Service has allowed an exception to its monopoly for extremely urgent letters.
Got almost no spam, maybe 10-20% of my mail, mostly sent by local businesses and free local newspapers.
It's not a monopoly. Forgot about UPS, DHL and FedEx?
Their employees are usually very hard working and have to deal with all kinds of assholes.
Plus, it's much cheaper than their competitors.