you haven't seen what happens when you ferment yoghurt for 3-4 days. it gets quite liquid enough. 2 days, sure, you can barely pour it out.
in serbia the most common form of yoghurt packaging is bottles with about 1" diameter openings. it pours out freely. typical ferment time is 2 days.
the fermentation process goes through two stages, the first stage it sets like a gel, but once it runs a bit longer, the protein is broken down more and it loses its cohesion.
it's actually better for you to drink yoghurt when it has been fermented like this but it is very sour. there is a lot less lactose and casein remaining in it after 3-4 days.
when i was a kid there was a brand of yogurt that was sold in giant 2L tubs, and it also was not very thick.