Mainly because we're in the era of tabloid sensationalism. You'll find people go to extremes in both ways on this forum. When we win a few games, people come out of the woodwork going through the upcoming fixture lists "If we just take 18 points from the next 18 and Chelsea drop 7 points in the same timeframe" etc etc.
On the other side of the coin, you have the people who will come out after a few bad results / performances.
There are some, then, who are in the happy medium. Like for instance, myself,
@Pexbo who started the thread and may others. OK, we're not delighted with the way the team is currently playing. But we believe in Van Gaal and realise the job that was on his hands.