You hit the nail on the head. Cable Internet infrastructure is the main problem in the United States. Most cable companies operate on physical cables that are 40+ years old and just cannot deliver higher speed Internet.
It should have been upgraded to fiber optic cable years ago, but it means a costly investment for them. When they advertise "fiber optic cable," it might mean they have it at some point in the line to your house, but not all the way. So basically it’s like taking a bullet train to the middle of Los Angeles, then being stuck in traffic for 2 hours for the last 10 miles to your house...