Why do car fuses melt and not blow?

The fuses are made of either a solder/lead type mixture, or it's a thin wire, the thickness makes the fuses melt at a faster or slower rate when they get hot from a "short" or too much draw for the circuit. When a fuse "blows" is just a saying. But if you put a large enough "short" on a fuse it will seem to "blow" when it melts extremely fast.

Also, to much resistance caused by dirty or loose connections on a standard plastic spade fuse can cause the plastic portion to melt.