Re: They do.
"That sounds much better than around here, where CityFibre have recently been along my street adding a new pole 5ft away from each BT pole. Most of the BT poles only appear to have a handful of cables on them."
Odds are that BT would require a rental fee to use the poles, ongoing, forever, which will have paid for itself after a year or two. Profit! So CityFibre looked at what BT wanted and decided to put their own poles in, paid off in "saved" BT pole rental in two years and so "savings!!" In the end, BT lost potential income and CityFibre spent more than they need to.
The obvious and fair solution, with wins all around, is for BT to charge half the new pole installation cost as a one off fee for using their poles. If a 3rd supplier comes along and wants in on the act, they pay 1/3rd of the new pol installation cost, again as a one off fee, but that payment is split two ways between BT and CityFibre. Cost of maintaining the poles is split between the poles users with the initial pole owner deciding what to do and when unless other pole users can demonstrate issues with the poles affecting their services. Same system can apply to underground ducts too. Someone pays once to put the duct in, any future users can pay their "share", as a one off for access.
Of course, the pole and duct owners can't then gouge the other suppliers, so of course it won't happen without regulation and/or laws and it goes against the grain of capitalism.