How did every animal on Earth evolve with 2 equal sides?

You're mising a lot:

5 Main Types of Symmetry Seen in Animals

https://www.biologydiscussion.com/animals-2/5-main...

As others pointed out below, many animals have different forms of symmetry, or no real symmetry at all.  But the reason a lot of them have a form of symmetry is: it's easy for genetic code to say effectively "do this on both sides of the body".  Why not 3 or 4 is a tougher question, but I can say our heads would get Very Crowded with 4 sets of eyes and ears, plus a nose and mouth.

First of all you are quite wrong about animals on Earth. Not all have bilateral symmetry. Even bilaterally symmetrical animals do not always have the list of parts you list or the number of them you list. You are plain wrong.

Your question may be affected, even if subconsciously, by science fiction. It would seem that the human authors of novels, screenwriters, etc. have a somewhat limited imagination and tend to make aliens humanoid, i.e. obvious body and head areas, four limbs, etc. It is perfectly feasible that life on other planets will look nothing like we do.

That works pretty well for the bilaterian clade of animals.

You'll notice that cnidarians (jellyfish) and comb jellies don't look like that.  Nor do placozoa nor sponges.

Even in the bilaterian clade you can find something interesting:  echinoderms (starfish, sand dollars).  These guys grow in imposed radial symmetry.

We have an interesting set of body plan genes.

You can't assume that extraterrestrials would have bilateral symmetry this way.  It works pretty well -- but a radial symmetry would also work pretty well for things that have a "worm" or "suspension feeder" lifestyle.