Because OFCOM are an incompitent bunch of useless tossers, and wholly ineffective at creating compeitition in the market, and so rather useless at their job they have recently had to have a name change (from OFTEL).
Now OFCOM, being the ineffective, useless bunch of tossers that they are, felt they needed to justify their existence. So did they try to stimulate competition? Did they break, the useless tossers. They said 'BT' were no longer allowed to run their own network. Why? break knows - they think that may somehow stimulate competition (I'll come back to that).
So, Openreach, are not allowed to be part of 'BT' (although financially they are under BT Group, they are a seperate legal entity and have to be run as an unrelated business). So, if you want to know why Openreach are not part of BT, its because OFCOM say so.
So, how will this stimulate competition? I think by ensuring 'BT' cannot do its job, and ultimately pissing off customers because this, people may go to another provider. Trouble is, the oppswits at OFCOM don't realise that outside of large towns, BT is the only option. So instead of a slick company being able to sort out issues, you have 2 that are regulated in such a way that they are simply not allowed to work together effectively.
So if you feel you need to tear someone off a strip, pick up your none working phone, drive to London, and shove it right up Lord Currie's jacksie, whilst calling him a pointless, incompitent, useless idiot.
And no, I don't work for BT or Openreach (used to though, so know the setup and the tight integration of the systems that now have to be pulled apart into 2 seperate entities).