You frequently hear in the news that the latest film is the biggest
blockbuster ever, earning record revenues in the first weekend of
release, and so on. Why is it that such statements are misleading?
What should reporters to report such events more accurately? Why don't
they do so?

The reporters aren't being misleading, you aren't listening carefully
enough. To say a film is "the biggest opening gross ever" is one
thing. To say it is "one of the biggest opening grosses taken on a
non-holiday weekend in winter" is another. Adding "one of" is very
popular when you need to pump up a bit of fluff.

If you listen to baseball you'll hear the most astounding statistics
and eventually will want to make up your own: most wins against a
left-handed pitcher that never went to a school with the letter "E" in
it's name, best at-bats during the month of August at ballparks where
the temperature is between 28 and 31 degrees C; winner of the
father-son north central regional mountain state amateur team
invitational in two non-consecutive even years. If you do this enough,
you can come up with a near-superlative for anything.

Now if you mean the entertainment "reporters" who are actually just
talking dogs, they do it because they're paid to.