Why don’t people flock to UPS and FedEx to send their personal correspondence?
Probably never thought of doing it.
Letters are so 20th century.
Because it is illegal to do so.
The USPS is a government created (and therefore legal) monopoly for delivering first class mail.
I remember a news story from probably 30 years ago of some guy in New York city who started a business delivering first class mail between highrise offices in downtown undercutting the post office rates. It didn't take USPS long to shut him down and put him out of business.