It's not really "extremely cheap" and probably only refers to covering old buildings with external insulation.
Most of the Soviet-era panel apartment blocks (https://en.wikipedia.org/wiki/Panel_building) have extremely shitty internal insulation, too. So you end up heating your neighbors, the street, the elevator shafts, stairwells etc.
You can't really fix that without extremely costly renovations.
Khrusschchyovkas (https://en.wikipedia.org/wiki/Khrushchyovka) are marginally better due to materials used, but they are 50 years past their demolition date by now, and will be also very expensive to retrofit.
There is also an insane number of new construction in the past 30 years. Perhaps for the in-EU Eastern European countries regulations and standards work. Everywhere else it's "whatever we build, as cheaply as possible"
I think having some insulation is better than having none. But yeah, if you want to do it properly, it would cost a lot more.
As for heating your neighbours, a lot of cities have city heating and is paid depending on how many people live in the apartment, so you don't care that much about it. Of course, some folks have gas boilers in their apartment (like me) but it won't make sense financially to insulate the inner walls.
I think it's the same for new buildings here too, if not worse. A lot of regulations aren't actually respected and because a lot of builders left for WE, there's a huge problem with finding skilled workers.
Most of the Soviet-era panel apartment blocks (https://en.wikipedia.org/wiki/Panel_building) have extremely shitty internal insulation, too. So you end up heating your neighbors, the street, the elevator shafts, stairwells etc.
You can't really fix that without extremely costly renovations.
Khrusschchyovkas (https://en.wikipedia.org/wiki/Khrushchyovka) are marginally better due to materials used, but they are 50 years past their demolition date by now, and will be also very expensive to retrofit.
There is also an insane number of new construction in the past 30 years. Perhaps for the in-EU Eastern European countries regulations and standards work. Everywhere else it's "whatever we build, as cheaply as possible"