Do they? They are from Japan, not Hollywood. Very different business culture. And while they are successful and beloved, compared to Pixar they only made small money. They only have one movie who made comparable money to a Pixar movies, which was Spirited Away in 2002/2003, and it "only" made as much as Toy Story in 1995, and a bit more than "A Bug's Life" in 1998.

So even if investors in Hollywood knew about Ghibli around that time, they probably didn't consider it as a relevant investment.