Yes you can build your own, but it's tougher to get right than you might think. Those tiny robots are incredibly sensitive to the conditions of the maze, and you can get things wrong quite easily. You could build the whole thing to all the millimeter tolerances, then learn that oh no, your white paint actually has some undisclosed additives that absorb IR light instead of reflecting it, meaning you have to complete re-paint and re-sand all the walls in your 9-square-meter maze. Oops.
You can do it, but it's still quite tough; in many ways tougher and less fun than building the mouse.
Why would you construct an entire 9 square meter maze without testing a small portion? If only for having a physically convenient way of quickly validating hardware and software changes?
Why would you paint the entire thing without testing the materials, or asking the organizers what paint they use / how to validate your own maze, or asking among fellow competitors?
You can do it, but it's still quite tough; in many ways tougher and less fun than building the mouse.
Link to some helpful notes on building your own Micromouse maze: https://micromouseonline.com/micromouse-book/mazes-and-maze-...