Is it? The 737 has been sold 12000 times, with thousands of orders in the pipeline. The A320 is not far behind, but only got introduced twenty years later

Even if you just consider military airframes, the MiG-21 has been built 11000 times, and is/was used by more than 50 operators world-wide, all of which have paid for the privilege.

Then there's the C-130, with 2500 units produced and operated by 70 countries.

The first American Fighter jet would be the F-4 Phantom II, with 5000 units built, and used extensively abroad.

The F-16 has 4600 units built up to now and is used by more than 25 countries.

(I'm ignoring the P-51 with its 15000 units here because they were mostly used by the US and rarely sold abroad).

tl;dr: The F-35 is not the most commercially successful airframe in the world, in fact, it does not even come close.