The fun question to ask is why almost all galaxies (no matter how they differ) spin as though a simple modification of gravity explains them. That dark matter is organising itself to produce the same outcome over many different circumstances!
They don't. You can read exactly this article for 3 examples.
Modified Newtonian theories are anything but simple.
You do make a good point though - astronomy and astrophysics is hard. There are often many ways of interpreting the observations we have. They can be misleading and take careful interpretation, particularly when they seem unusual in some way.
Visually galaxies are quite varied, which implies, to me, they have a lot of variability in visible matter. Why wouldn't their supposed dark matter vary too?