They're thinking they need to provide ventilation so it can dry out.
I think industrial designers should be required to live with the product they've designed for two months of regular use, maybe more, before they submit it for production.
And it is true that mine, which is solid (though there are certainly ways for air to get in there) does sometimes still have a little puddle of cleaning solution in the bottom by the next time I use it, because the brush holds onto a lot).
But I never worry about that cleaning solution being there; I'd rather have that than have to wash the holder. Inside the cabinet, it wouldn't be as bad because it wouldn't get dust (skin cells, terry cloth fibers and cells, toilet paper residue and cells...) landing on it. But...