I think famous western young adult novels have three or four volumes.
But Japanese light novels often have 10 or 20 volumes if they sell well.
Of course, my question is the same as why Prison Break and 24 have many seasons. Economy!
My true question is that why famous western YA novelists don't have to write many sequels.
First, English users are 1.7 billion if I remember correctly. Japanese population is 127 million.
It's more difficult to sell 2 million copies in Japan.
Second, economic ripple effect is completely different.
Famous English novels often publish in other countries too. In Japan, they use advertisement like "Best seller in America !", but the opposite is very rare unless the name is Yugioh or Pokemon.
Third, western authors tend to advertise their names besides their books.(To me, it's very interesting to see books with the author's name larger than the title.)
Japanese authors are not willing to show their face and information.
I don't mean they don't have any name value but this difference is big.
Fourth, western authors and publishers have a more equal relationship.
LN authors tend to feel giri
(obligation or appreciation) to publishers and take many requests.
My opinion must have some generalization, but I feel this way.