They play games with the laws. There is no exclusive franchise, but they do make right of way laws that make the barrier to entry incredibly high. AT&T successfully sued my state government to disallow ISPs access to right of way easements. But you ask, isn't AT&T an ISP? Nope, they're a telcom, completely different. They offer internet services, but they are not an "ISP" by right of way definition. Funny how that works. AT&T wasn't alone in suing the government, so "cable" companies also have access, but not ISPs.
Then you couple that with another common city law that states that unless you have right of way access, there is a limit of how many companies have access. They don't want lots of competition coming through and tearing up everyone's lawns. So we have this law that effectively limits the number of ISPs right of way access to exactly 1, but AT&T and Charter aren't ISPs, but because they offer ISP services, they still count towards that "1". So yeah, good luck.