Playing through Wario Land 2 again, I can't help but wonder...

For the same reason Princess Peach's castle is filled to the brim with stairs, long hallways and pictures that don't make any sense at all: because it's a video game and all these random things are just there for the sake of the plot.

Canonically: maybe Wario had indeed a really big bathtub once there, and enjoyed swimming in it.
 
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