MathML as a spec is akin to SVG, it's meant for the browser/machine first, and not for direct human consumption. You don't read/write SVG by hand, and neither you should MathML.

That's true, but the problem is that MathML is fairly useless without a robust set of tools to manipulate it and some way to do efficient manipulation by hand. These tools didn't develop in any meaningful way and so people haven't adopted it.

MathML by itself is only half an answer without a way to easily convert a more human friendly format bidirectionally without loss of detail.