That’s a chicken and egg situation. The costs are fixed because they have no incentive to build more infrastructure.
If you live in a place with only one or two ISPs and they only charge a flat rate per month for capped usage, you never get more bandwidth. The system is working as designed.
If you live in a place with only one or two ISPs and they only charge a flat rate per month for capped usage, you never get more bandwidth. The system is working as designed.