Which is fair-- given some paint and masking tape anyone could copy his work. But that's not what's important, not the technical aspect anyways. Really, it's a fascinating body of work because of its CONCEPT.
(and here I want to note that this is based on what I have read and MY interpretation! If you had a different idea of his work I am not discounting it... that's the fun part about art, you can interpret it in any way you want)
Mondrian's work is about the ideal of beauty. For a painter planning out a painting, he/she has to consider how to break up a canvas. There are ways to do so the create asymmetry, balance, etc. to make it aesthetically pleasing. He does this through carefully calculated geometry and methodically placed lines/blocks of color.
Mondrian said that you don't NEED to have a subject like the Mona Lisa or complex ranges of colors like the impressionists to make a pleasing painting. If you break down art to its most basic components-- line and the primary colors and negative space (the areas where there isn't)-- and arrange it in one of the universally pleasing compositions you will still be able to achieve the ideal of beauty.
That idea was incredibly revolutionary!
Then consider, what would be the point of that? Well by simplifying it, by removing distractions of narrative and recognizable forms, you can try to understand purely the ideal concept of beauty. Beauty not being a thing that is just in the world, but in its purest form or as a kind of force of nature or universal truth... Plato in his theory of forms argued that these kind of universal ideas (like beauty!) is the only reality. The search for truth, a push to understand reality, drives all art
More concisely Mondrian was getting rid of distractions (back story, character, value-- who needs it?) to only try to find what beauty really is in the universe.
The pursuit of beauty? That sounds a lot like art to me