That might be true, but you are ignoring that there can be cases when it isn't even clear who the most valuable player is. If two or three players are neck and neck in terms if performance and/or importance, then it basically becomes a coin toss. Under those circumstances, since a voter has to choose, he might go with "one has already won the trophy and the other hasn't, so I'll pick the latter", especially when the latter has been among the best players for years.
In other words: it isn't a case of a voter ignoring performance just to hand a player he likes an award. Instead it's coming to the conclusion that there is no right choice, and thus going with what feels right. In this particular case someone might think that a) McDavid has won often already, and hasn't seperated himself like he usually did in the past, b) that Kucherov has won already as well, and c) that MacKinnon has not and that you might never know if he will be in such a position again. Hence choosing option C among three basically equal choices.
In the same way one could instead choose McDavid, because he is on par with the other two even though his first month got ruined thanks to an injury. Or because what a difference it made in how his team performed while he was injured / played injured and how it performed when he was healthy.
Or you could opt for any other reason, as you can always present something in a way that suits your logic. E.g. picking the one whose team is highest in the standings, or choosing the one whose team is lowest and arguing that it is only due to this player that they are even in the playoffs, the list goes on and on.
If all three stay this close together the rest of the way (and Tampa doesn't miraculously drop out of the playoffs) than all three are good choices. And as much as some people will claim that "X got robbed" if their favourite doesn't win, that simply won't be true.