The options aren't "conspiracy theory" on the one hand and "well-meaning professional types doing a difficult job under a lot of political pressure". There's a fair bit of in-between, as well as different ways of looking at it entirely.
Political journalists are typically far too close to their subjects, socially, professionally, politically and demographically. They all went to similar schools and universities and believe the same things. They have always known each other: many are married to each other or otherwise related. They all go to the same parties. Hacks rely on Downing Street for access. Even in normal times there is a revolving door between the BBC and CCHQ (current standard bearers: Robbie Gibb, ex-head of BBC politics, now Downing Street director of comms [and brother to Conservative MP Nick Gibb]; Allegra Stratton, ex-political editor of Newsnight, now Downing Street press secretary).
On top of all that there is always political pressure related to funding, all of it from the right. And almost all political journalists are fantastically self-regarding and incapable of self-reflection, so they don't feel it necessary to account for any of this in any way, or develop formal rules to manage the overwhelming pressure to fall in step with their subjects. I mean the idea that there could be a meaningful level of objectivity under these conditions is absolutely laughable and the only people who actually believe it are the hacks themselves. And in fact there isn't any objectivity is there: you just need to look at the things they do and say. All of its got worse as the Conservative Party has got worse, the journalists more thick and the BBC's position more (apparently) precarious.
Where's the conspiracy? It's all out in the open and they don't even see any need to defend it. Just look at how people like Owen Jones get monstered when they suggest that, e.g., Allegra Stratton moving from the BBC into Downing Street, while being married to the editor of The Spectator, which used to be edited by the current prime minister and is owned by a company chaired by the BBC's Andrew Neil, is maybe a little bit off. They literally can't see a problem!
I'm not. As I've said, I've considered all that, weighed it up, and decided that it's not worth the price we pay for it, which is a one-party state.