Being a ref, or a VAR ref, is not so different to many other jobs. People choose the path of least resistance. If you need to do, or not do, something at work to stop a difficult customer going ape, you do it for a quiet life.
So if Andre Marriner thinks he'll have the Daily Mirror, half of Sky Sports and Jurgen Klopp screaming blue murder for not reviewing Casemiro's slightly high challenge, he'll choose the path of least resistance and review it - even though he's not supposed to re-referee something Taylor saw clearly. At worst he'll have a kindly colleague have a quiet word in his ear for not following protocol, which is definitely preferable to having the press on your back for weeks.
Likewise, easier for Marriner to just ignore the penalty appeals as by doing so he'll avoid most of the flack.
Taylor, well the scousers and their pliant friends in the media did a fabulous job of planting a story out there that he might be a Man United fan, so in his head every decision he gives in United's favour, or doesn't give in the oppo's favour, will inevitably lead to this being raised again.
To be honest I can see that it won't be pleasant for them.
It also shows that the refereeing bodies are doing an appalling job of (a) defending referees for making correct but massively unpopular decisions (think the Rashford goal v City, the refs were just hung out to dry whereas they should have been very publicly backed) and (b) holding referees to account for the mistakes or omissions they do make.
They really must do much better.
And as for United, I really can't see any alternative but to play the media game just as hard. Put pressure on the referees pre game, very publicly state that referees have one job, which is to call the game correctly, regardless of how the media might report it. Put pressure on the refereeing bodies.