As a member of the Mapping Team
To be 100% the reason there are not toilets in every building or even the major ones is primarily due to the engines memory limit, when making a map there are only so many brushes (things like walls, floors, and cellings) that a map can take, and this memory limit is not very high since it was set in 2004 when half life 2 came out I from what I know has not been updated. Now it is due to this that adding bathrooms on a map already filled to the brim with buildings, roadways, and other structures that are made of brushwork that adding bathrooms would be a unnecessarily building feature that not many people would use and it would take up even more of the maps memory limit, as of now we the mapping team are ensuring that we don't overdo this limit and totally fuck the map. So sadly this really can't happen.
If you our asking literally just for some public toilets they may be like one but its not a large thing spread across all the buildings.
It would be cool but just really cant be implemented due to engine limitations, sorry.