Because it's an isolated demand for rigor and gives away the real game which is "take out the target".
If flipping the partisan lens helps, consider the Hillary Clinton 2016 emails thing. Did anyone really care about the emails? Did we have immaculate email security from 2017-2020?
This seems like a really myopic view, focused on ideas like blame and purity.
Or, another way, this seems like a blame the messenger strategy. Sure, great, Media Matters is not a perfect arbiter of whatever. But how does that make it not a problem that Ford's ads are showing next to overt Nazi content? Am I (or is Ford) supposed to handwave it away because the same thing could happen on another platform?
Henry Ford has been dead for 76 years and the company is not currently run by vocal antisemites, so I don't quite get the point you're trying to make. Presumably you're saying it's not a big deal and they don't really care?
Recognizing an isolated demand for rigor isn't necessarily a demand for purity.
It's specifically noticing when nobody cares at all about a topic in all contexts except when it gores a particular ox.
In both this example and the emails thing, you never hear anybody talking about it except some very particular cases where they're clearly motivated to tar a particular target.
"We all have to consider this topic to be very important, specifically and only when party X does it, and talking about anything else is whataboutism!"
If flipping the partisan lens helps, consider the Hillary Clinton 2016 emails thing. Did anyone really care about the emails? Did we have immaculate email security from 2017-2020?