> why almost all galaxies (no matter how they differ) spin as though a simple modification of gravity explains them

They don't. You can read exactly this article for 3 examples.

Modified Newtonian theories are anything but simple.

If you read the rebuttal by Professor McGaugh, you can include those 3 as perfectly good examples of MOND too.

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.