Boom are flying with turbofan engines, not the turbojet engines that were common in the 60s.

As for aerodynamics, the F-22 is relatively efficient at high subsonic speed and also doesn't require external tanks like earlier jets do.

Wikipedia says that the F-22 is able to supercruise without afterburner, using turbofan engines. This may be the key.

Supersonic capable turbofans that are able to get the "force multiplier" of using bypass airflow for thrust at supersonic speeds are honestly the key enabling technology here.

So basically a supersonic turbofan is a SCRAMJET behind the first fan?

Not being an expert on supersonic turbofans, I went to the obvious first information source and it was surprisingly illustrative. https://en.m.wikipedia.org/wiki/Turbofan

From a little further reading it seems it seems that supersonic turbofans are actually just the apex of the continued sophisticated work being done on jet engines which has allowed them to push the intake fans of low bypass turbofans up to supersonic airflow speeds.

As is the Typhoon

Why has nobody designed a variable width wing? One that can expand / contract for different speeds?

Naïve question, I know. Just curious...

They're called flaps and slats.

https://www.grc.nasa.gov/www/k-12/airplane/flap.html

Barnes Wallis developed some interesting designs in the early 50s: http://www.sirbarneswallis.com/Supersonics.htm

Concorde has a variable width wing.

I think they mean variable geometry more in the line of the B-1 Lancer or F-14 Tomcat... And the answer is probably cost, complexity, reliability and weight.

All four of those.