Because F-5 was developed as cost effective aircraft with the simplicity in operation in mind. Something that is rarely, if at all, done today (at least successfully). It was mostly developed on private founding of Northrop.
Also it isn't falls to the same category as Scorpion (F-5 is in the same category as MiG-21 or Mirage III) but the general idea behind the development are very-very similar.
My original statement was kinda jokingly stating the obvious. The two have very little in common. The original F5 was built with low risk due to USAF buying the T38 trainer, 1200 of them. Most of all there was no question the market was there for such a fighter. A supersonic interceptor able to mix it with the best of them and with good ground capabilities. This fit the bill for many, many countries on limited budgets during the fierce and uncertain days of the Cold War.
All the Scorpion seems to offer is modest ground attack capabilities. Even its recce potential has been weakened by the age of UAVs. So that leaves search and rescue since there aint a whole of Cold War Bush wars going to sell its modest ground capability. And there are only so many rich Arabs who want a 2 man jet to zip around the desert in.
It just strikes me as the wrong jet at the wrong time in the wrong world situation.