I can't comment on the installation of mains sewerage but I imagine it is very similar to how mains electric is, or at least used to be, installed, and I am only grumbling about installation, not subscription costs as those have always been backwards with BT only exchanges paying the highest fees and therefore subsidising those exchange users with competitors.
Openreaches "problem" is that they charge a fixed price for a line install, regardless of actual cost. Mains electric certainly used to be charged based on distance and other factors, like having to lay it underground in conservation areas. So if your phoneline actually cost £1000 to install, you paid 10% of the cost, if your electric lines cost £1000, you paid 100%.