Because in itself, it is a lie. It leads to vain imaginations. They imagined words like galaxies and nebulas, and all sorts of stuff, but their is not actual proof if anything they say is true. They add colors to enhance the illusion. Everyone believes all this stuff because of star trek, doctored photos, movies, and so on.
I like the photos, but you can't actually look at that and divine how the universe works or what that stuff is.
The scientists who've studied the cosmos for most of their lives build on what those who came before them did. They test the theories and do the math, and when they're satisfied that the theories are sound, they branch off to do their own work and create theories of their own.
Of course it's possible that there are explanations other than what mainstream science has conceived, which is why it keeps evolving. Peer reviews are there both to back up and disprove hypotheses, and no matter how long and vehemently a theory has been supported, if someone comes along with a better explanation, or even shows why the current theory is flawed, the scientific community doesn't ignore them.
There are things in the universe that were mathematically predicted, using Einstein's Theory of Relativity, before they were ever physically witnessed, such as black holes.
If people wish to just dismiss things, simply because they either don't understand and can't imagine anyone else could, or because it flies in the face of everything they've believed (without any proof whatsoever) that's their prerogative, but when they make unsubstantiated and baseless claims of foul play, forgeries, and disinformation in an effort to further their point of view, I have no respect for their assertions.
What possible reason would the scientific community have for making stuff up? Why would all of them play along?
When someone says, "prove it", then claims the photos are CGI, what more can anyone do to show them the truth? There are some people who could be taken into space on a ship and they'd say it was all an elaborate hoax, perpetrated to keep them in the dark.
At some point, people have to just realize there are other people who simply know a lot more than they do.