Because you're trying to enact policies on the US in an official capacity without actually holding the position that gives you the authority to do so. For example, he met up with world leaders and was talking about policies to implement in the US; he was not technically president when this was occurring. This is illegal.

But of course, Democrats are above the law...

EDIT: I should also point out that the "Office of President-Elect" literally never existed before, and it is not a federal entity of any kind. This by itself makes what he was doing illegal. They only made it to normalize in people's mind that he was "president", even though he really wasn't; it was more of a psychological operation, I guess you could say.

Because you're trying to enact policies on the US in an official capacity without actually holding the position that gives you the authority to do so. For example, he met up with world leaders and was talking about policies to implement in the US; he was not technically president when this was occurring. This is illegal.

But of course, Democrats are above the law...