Because even if they actually had an income of zero, they had saved up enough money over the years to be able to survive off it for some time. Commonly referred to as a “war chest”. They don’t actually have an income of zero though, their income is actually quite significant since they still have major trading partners like Iran, India and China to sell their fossil fuels and other resources to. They don’t buy fossil fuels, incidentally, they sell them. Like Saudi Arabia.

Also, you can buy things with more than just money. People figure that Kim Jong Un probably isn’t trading his stuff to them for money, but instead is getting technological assistance from them. Just one of their modern fighter-bomber tech, for instance, would be immeasurably valuable to N Korea, where their own tech has lagged behind a good bit over the years.

Lastly, people in the west have been doing sanctions evasion. I recall some German financial/tech company has its CEO now wanted by Interpol for being a Russian agent. So, when your own people are playing fast and loose with the law, that’s going to make things like sanctions more difficult.