I mean, if they were reporting on Biden's progress in governing through that lens, and Trump's progress in the election, then I could see validity to what you're saying as the reason why. But that's not the case -- they're reporting the election in those unequal terms.
One great example is the little nugget contained in his answer, where Trump is "winning primary races" and that's a notable point about his popularity. Biden's won 86% of the primary popular vote. Trump's won 72% of the primary popular vote. Every single person who follows political news knows that there's a little revolt of uncommitted voters because of Biden's support for Israel. How many people know about 30+% of voters in Republican primaries saying that they won't necessarily support the eventual nominee in November? That's very unusual, and clearly a bigger story on exactly the same subject, and it'd be worth diving into the reasons behind it because they would uncover some objective things underlying their decisions that would be great to report on. Yet somehow it gets less press than the uncommitted voters making problems for Biden (which, obviously, are also an important story to report on.)
I personally don’t find this “the media is so mean to Biden!” narrative any more compelling than when Trump was claiming the same thing as president. The media has always been critical of those in power and this is a healthy part of our democratic system.
I mean, every president in modern history has whined about how the press is being mean to them (usually with some validity). It's part of the job. But it doesn't mean that careful analysis of "is the press coverage actually slanted" suddenly turns into an automatically wrong thing.