Competition is hard, but not impossible. Our main problem is that we write large quantities of laws and regulations making the barrier to entry even higher, but don't prevent large companies from simply buying local networks to expand into new markets. Comcast, like most large ISP's, didn't expand, they purchased. They do massive infrastructure work in the form of upgrades, but most of their market expansion was through purchasing existing networks.
Setting up even a local ISP costs a lot of money. If you're running fiber through suburbia, you can spend a thousand plus a residence just to lay the ground work. Yes, it really is that expensive. If there is already a competing network and you expect to get half the people, you're doubling the payback on that initial cost. It's estimated that Google blew a hundred million bucks to get fiber up in Kansas.
Wireless can be done cheaper, but then it also has more issues. There are only so many frequencies to go around, competing wireless networks can interfere with each other and saturation problems are exacerbated by the fact that all those wireless signals are mashing together and leading to data corruption. They end up being much slower than they could be once you get a handle on all the issues.
The entry barrier, even in a perfect free market, is exceptionally high. Which is why only major cities have great internet, they're the only markets rich enough to be attractive targets. A city under a hundred thousand is never going to be a good market, and rural America is just plain fucked.
I'm actually against disallowing a national competitor from buying up networks to expand into monopoly situations on principle alone regardless of the effect, but an argument can be made that the FTC doing it's legally obligated job would help with this by forcing companies like Comcast to compete to grow their user base, instead of simply replacing existing providers. My expectation is that it would indeed lead to more competition, but that the realities of the limitations on a monopoly would result in better long term advancement, than the reduced margins from competition would be able to pay for.