The buildings are concrete and poor isolated, single windows. Old as well. In cities they get very hot, hard to keep them cool, unless perhaps in lucky cases where all your neighbours already run full steam. On top of often older aircons, or higher than government electric rates.
6K is certainly over the top, but it is no surprise to get 2.5-3.0K baht electric bills at a 4K room. Unless you love to heat. Also, not so much about it being a 4K room but more the general size of a studio, would be same at a 12K condo new building studio size (which are also concrete and poor isolated).
Mind you that the 4-12K rates usually are 3+ month contracts with some deposit.
Aside all this, transport costs also add up, easily gets 3-5K per month. If you factor in all these variables, it is not that bad of a deal to be on walking distance of everything, without deposit or contracts, at just 15-20K a month.
But then again, this is why ''what you like'' matters, some people would rather be 20 mins out of town for < 5K rent so they can then rent a big bike for 5K too, and enjoy 5K on fuel to drive around.