I'm seeing a lot of comments on Biden here and a few theories being posited as fact, but let's actually try to look at this critically. The OP asked what I believe is an important question. Let's try to solve it.

I want to be clear, I don't know the answer. But let's start looking at brass tacks here and look at possibilities and distinctions, including prosaic ones:

-What are the potential reasons that Biden can't or won't use the Oval Office?

-What benefit(s) is there to using the fake set?

-Is the fake window meaningful or simply window dressing/red herring?

-Is there any logistical or general advantage to Biden and his handlers when using this fake set instead of the Oval Office?

I think these are some of the questions we need to start discussing. It's very, very strange that he's been using this fake set for so long and I think it is significant. But we have to break it down in a clinical fashion. Does no good to post wild theories.

EDIT: Some additional information I was able to find:

-The fake set is not even in the White House. It's located in the Executive Office Building which is used for Executive Branch staffers.

-Martha Kumar is one of the comms directors for Biden and she states they're using it because it's "more accessible". I call bullshit on this.

-At least one "insider" has told a couple of news outlets that the reason for the fake set is "entirely about optics". I call bullshit on this as well. The fact that he's not in the oval office is a distraction from whatever message he's tripping over. And more importantly, it makes him look even less Presidential than he is already.

I'll continue to add any information I find if people are interested in discussing/solving this.