I assume you are talking about consumer ISPs like the ones coming into your house. And also you are talking about the US.

Money is the least of your problems but a big one. The other ones are regulations, land, and incumbents.

Current US isps are setup because the government made a deal back then saying they would get sort of a regulated monopoly for exchange of helping build the network. They also got money to help with building the network. Now that they have a monopoly they will not like having a new person come in. See what happened with google fiber and their fight with att for telephone pole access. Or muni ISPs. Basically the incumbents ISPs will use current regulations to keep you out. They will even drag their feet if regulations are not in their favor. See the telephone pole example again, google wants to hang stuff off those poles and it is a-ok with the government att comes in and say "oh shit i left those in a mess let me clean it up for you before you go hang your stuff up." What they don't tell you is that they will put those work orders as super low priority and reduce work staff so nothing gets done. Comcast was even accused of cutting their competitor's cables.

Man sharing poles is hard why don't I just put one up myself. Well good luck convincing every other homeowner to let you plop a pole in their backyard and dangle wires from it just so you can make money.

On a side note a few companies are trying to avoid this "land" fight by deploying wireless internet for you home. One example is https://sailinternet.com/ and we will see how that goes for them.

Now lets say you got your physical network all rolled out to your consumers. You still need to plug it into some other network that is connected to the internet or else you are not an ISP. This usually requires money or some other trade of equal value. Once all that is setup you need to do traffic management on the network, maintenance of infrastructure, security, billing, customer support, and marketing.