Because for profit companies want to make as much money as possible and spectrum is limited. Which is more or less why buying an empty field in the Brazil is ridiculously cheaper than the same sized empty field in downtown London.
You don't need to start by building a nationwide network when you start out. 1 Cell tower can handle a little over 1200 calls at the same time and cover up to 1,500 square miles using 2002 teck. So start with one tower in the major city(s) for a few million and add towers as needed after that with the revenue from customers.
Which is why there is a cell network in every country. The problem is a few towers is not going to cut it once someone is covering the country.
PS: Building a modern tower in the US costs ~300k USD as a baseline including equipment and construction costs (though you can have more than one network per tower). Covering the UK wold cost at a minimum ~30million USD for the towers assuming ~1k square miles per tower. Less if you include significant gaps, laying the ground network is adds costs. But you would probably pay 10x just for the spectrum and you would want a much higher density in city's, and land is not free etc.
Or why doesn't BT in the UK just build its own mobile network instead of buying EE