It is _amazingly_ expensive to start building a network.
There's about 4 vendors providing the gear needed for a large scale cell network. And they're not bothering talking to you unless you want to buy a lot of gear.
Then you need spectrum licenses. And since you're placing cell towers around, you're either going to rent rather expensive towers, or build your own, which requires permits and a lot of civil engineering.
It's a bit easier today, as there's more infrastructure in place - but you may very well end up needing to provide the network backhaul from a cell site, somehow get power lines to the cell site. And that cell site might need backup power, which require refueling and maintenance every now and then, you might need to build a small cabin to house the gear, and you might need to hire helicopters to fly in that gear.
When you get this far, you need a marketing department, so people can actually buy connectivity, refill cards, sim card distribution, and similar.
At some point you might need roaming agreements with others as well. If your're lucky the government requires you to have agreements with other operators in your country - important enough, cause unless you have greate coverage people won't bother if they can't use their phone. If you want your customers to use their phone while abroad, you get to make individual roaming agreements with all those other operators in other countries.
