Why would they? Even in the days of PTEs having their own operator other operators still provided services extending all the way to the city centres. The free market model of deregulation may have lost credibility now but that doesn't mean there is support, whether from full-time politicians or the electorate more generally, for the creation of monopolies controlled solely by politicians or their appointees.
Of course cross border services mean you'll never get only one company in an area. But PTEs didn't have the benefit of being able to take over all services in their area, did they? If they wanted to take over another operator, they had to do a deal and get the cheque book out and have a willing seller. In Greater Manchester, Lancashire United sold up, but Mayne resisted all attempts and remained running under the agreements they'd made with Manchester City Council pre PTE.
Why do combined authorities like Greater Manchester and London, use the franchise model rather than just 1 company operate all bus services?
London's a different system. All the depots were sold off, and TfL tender services out. That's basically how it was set up by the then Conservative government, and that's the system that the Mayor inherited. TfL did directly own one operator, East Thames, but Boris Johnson sold it off.
And there's ultimately your answer. It comes down to politics, and the beliefs of those in power. The Conservatives wanted the private sector running things, hence selling off the PTE operations, London buses and the NBC companies. And also why they banned new municipal bus companies recently. Labour are less fussed hence why they're changing things so new public operators can be created. But, of course, there's nothing to say things won't get changed back in the future. New publicly owned companies may well find themselves sold via government dictact in the future!