Which is why ACA also instituted marketplaces and insurance standards, so that competition across substitute services keeps premiums competitive.
E.g. Anthem is more than free to double the premium on their bronze plan to increase net profits, but then customers are free to swap in bronze plans from Cigna, Kaiser, or any number of competitors that didn't.
Networks complicate things by reducing demand elasticity.
For example, look at Maricopa County, Arizona. It's like 100km across and includes both a major capital city (Phoenix) and several suburbs with 6-figure populations (Mesa, Chandler, Glendale).
Many of the marketplace health plans* would have networks which focused around central Phoenix. If you wanted to see an in-network doctor or urgent care and lived at the fringes of the county, it was a 30-minute drive or more.
This meant people in the fringes had to pick the few plans that covered nearby facilities, even if there was a significant price difference.
* This was during the first few years of the marketplace. By 2017 or so they were basically down to a single insurer, so even that degree of consumer leverage was gone.
E.g. Anthem is more than free to double the premium on their bronze plan to increase net profits, but then customers are free to swap in bronze plans from Cigna, Kaiser, or any number of competitors that didn't.