The problem is the last mile is a natural monopoly of sorts. If you have been to other countries you will often see telephone poles thick with wires from many companies. Abandoned wires from defunct companies, companies "accidently" damaging other cables, and generally a giant mess. That is the effect of a completely deregulated last mile.
A single fiber plant is that middle ground. For it to be fair everybody has to pay the same per wavelength and those proceeds have to go solely to maintaining/paying for the plant. Plenty of ways to structure that so it's not a potential hidden tax or discriminatory to a specific provider. An all optical plant means any company can put pretty much whatever tech they want, the last mile plant has nothing smart hell nothing that even needs power. Fiber from the 70's would serve just as well as fiber from today with no breakthroughs that look to change that basic fact. 80gbps is today's cheap bandwidth on a given pair (8x 10gbps) and 2.4tbps (24x 100gbps) with purely passive optics in the last mile.
A pipe per provider is self limiting as for each provider you need yet another pipe. A tunnel might fit the bill with similar issues to poles and is massively expensive up front. A single plant reduced the friction of a new provider to servicing the area. More providers means more competition. It also lends itself to meta providers to provide the rest of the infrastructure and backhaul (similar to today's DSL resellers just hopefully less broken).