cpu – Why multithreading isn’t everywhere?

Not almost, but all modern CPUs have multiple cores, yet multithreading isn’t really that common. Why to have these cores then? To execute several sequential programs at the same time? Well, when calculations are complex (rendering, compiling), the program is definitely made to use advantage of multiple cores. But for other tasks a single core is enough?
I understand that multi-threading is hard to implement and has drawbacks if number of threads is less than expected. But not using these idle cores seems so irrational.