Hi welcome to the fourm.
As Openreach provide our lines, this is the time frame we give to all our customers most providers (apart from virgin) have this time frame, to keep everything in check with the exchanges and ports your Internet is going to.
Exchange side lines though are in pressurised ducts so it takes alot of work to open them up. Also Exchange side cables can go upwards of 3600 pairs and jointing them can be complex and extremely time consuming. Openreach need to be contacted in advance to scedule each individual home with Broadband and Talk lines, even if you already have a line, the line is being swapped into another port if you switch provider and there is a list of other people waiting hence we cannot pirotise customers that easily ( as much as we would all like to )