The game already explained the bathroom thing. Monster food doesn't cause you to expel anything after you eat it, so there was no need for bathrooms.
It’s true that monster food absorbs converts perfectly into energy! This is why there is no need for a single toilet anywhere in Undertale. However, where would a human fit into all this? First, here’s what we know.
MONSTER IN GRILLBY’S:
Hmmm… Isn’t human food different from monster food?
It does things like “spoil.”
And when you eat it, it passes all the way through your whole body.
I’d love to try it sometime.
MONSTER IN GRILLBY’S:
And I’ve heard they have things called bathrooms.
MONSTER IN GRILLBY’S (after Undyne battle):
To a human, monster food would be very interesting.
As soon as you eat it, it converts perfectly into energy.
…that makes me wonder. How do humans do it?
Eat during battle…?
Ice cream in one hand, sword in the other.
That’s the way of the monster warrior…
MONSTER IN GRILLBY’S (after Asriel battle):
Soon, I’ll be able to try some human food…
And then I guess I’ll try a human bathroom, too.
Toby didn’t cover his bases very well though. Despite this monster never hearing of a “bathroom” before, bathrooms are mentioned a few times in Undertale.
Jerry tells everyone it has to go to the bathroom.
WHOOPSY DOOPSY! I JUST REMEMBERED!
I HAVE TO GO TO THE BATHROOM!!
Now we’re officially friends! Ehehehe!
I’m going to the bathroom.
I’m going to the bathroom, so I’ll be MIA for a bit.
I’m… I’m sure you can handle this puzzle yourself!
Considering that showers are mentioned by both Sans and Alphys, chances are these bathrooms are literally for bathing or showering (Alphys even has a tub in the True Lab). Toilets don’t seem to exist, especially considering Papyrus’ phone call:
WHERE ARE YOU? I HEAR WATER.
ARE YOU IN THE TOILET?
ALSO, WHAT’S A TOILET?
NARRATION (during Shyren battle):
Sans is selling tickets made of toilet paper.
Sans, why? Toilet paper could come from the dump, though that doesn’t seem hygienic. But this leads to the next question: how would Chara get by in a toilet free world, and what effect would a monster food diet have on them?
I admit that when it comes to Caretaker AU, I have given this more thought than is probably necessary. I’m not a doctor, but I do have a basic understanding of physiology so please bear with me and my reasoning.
Humans have very complex systems for absorbing food (digestive system) and for keeping the blood clean (urinary system). Chara would not be able to survive long without water, and even magical water wouldn’t cut it: kidneys create urine in order to get rid of waste built up in the blood. And while a lot of this is metabolic (food) waste, the kidneys also clear out other waste such as dead blood cells. Urine plays an important role in staying healthy, which goes beyond just food.
As for the digestive system, food with perfect absorption into pure energy might actually cause terrible long term effects. While I don’t have any data on what happens if the lower intestine stops being used completely, I can say that an organ that stops being used has a tendency develop problems and/or fail. With how dependent humans are on regular eating cycles, I would be concerned that instantly converted food in the stomach would end up causing the the unused lower digestive tract to stagnate and even fail.
Therefore, while the monster in Grillby’s assumes that humans would experience the same perfect conversion from food into energy that monsters do, I theorize the experience would not quite be identical. Monster food would be immediately energizing and healing with a very efficient absorption rate, but some waste would still pass through the body, due to a non-magical human body being unable to convert all of it. Small amounts of monster food would still pass through the entire digestive tract, ensuring all organs are still functioning regularly.
And that’s why in this AU some buildings had to be outfitted with toilets in the Underground for Chara.