So many polygons for such shitty graphics, honestly, minecraft could be developed with half or less of these polygons.
Issac already mentioned the base of it, in that you need a certain number of triangles (barest form of polygon and often the only kind used in games, higher-sided polygons are more complex to handle and often used in just modeling and things will be converted to tris upon export) in order to display regular geometric shapes, and Minecraft has a lot of blocks going on in the terrain.
Every single block face exposed is two triangles. That really adds up.
For comparison, here's a diffuse+wire render of a piece of landscape I built in
Blender, where you can see how many polygons are needed for smooth shapes and hills when you have proper sculpting tools and are pre-creating land. This is the sort of thing you'll see in games with more realistic terrain.
The hills seem relatively smoother, but are also using a lot less triangles, and so there's way less complex terrain to render.
Having low textures, but a high amount of polygons seems at best controversial
And using Amiga-style graphics on a modern machine may seem controversial, but
VVVVVV went ahead and did it just fine. Minecraft uses the style it uses on purpose, just as well.
You said exactly the opposite. It was made in Java for been easily portable.
It was two statements.
1 - They wrote it in Java to make it portable across computers, and it is (Windows, OSX, and Linux).
2 - They had no intention of it being on consoles when it was originally made, so portability outside of Java's domain wasn't even thought of.
Java is just the tip of the iceberg
Of your lack of knowledge on the subject, yes.