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.
Because he is rich and eccentric?
If i had so much money i would probably install an giant washing machine that is a gallery that is filled with nothing but portraits of Tom Selleck.