An NHK easy news article used １兆の４０万 for 4*10^17 (400 quadrillions). According to wikipedia,
１兆 (ちょう）is 10^12,
１京 (けい or きゅう）is 10^16
So they could have written １京の４０ (40 * 10^16) instead of １兆の４０万 (400,000 * 10^12), which both give 4 * 10^17.
Why haven’t they?
You’d write ４０京, rather.
They didn’t, because although those higher numbers exist, it’s extremely rare to see anything larger than 兆.
You can see it in English, too - newsreaders are more likely to say “four thousand trillion dollars” than they are to say “four quadrillion dollars”.
It’s also more dramatic to say 400000 times [an already really huge number] than just [another really huge number], because people tend to have trouble imagining numbers over a certain size anyway.