LogFAQs > #960860794

Because most anime isn't successful for a long enough period. The wildly successful series keep going and might get a sequel series / spinoff, but most don't make it. As for the example of Death Note, it had a movie or two, and then spinoff manga.

Keep in mind that most anime only lasts a season. It's a very volatile market. Some series wind up having very large gaps in production,

And most anime spinoffs would have to be proceeded by a manga spinoff. The problem there is that most manga are spearheaded by a single creator who also handles the artwork, which means they can't just run two series at once much of the time. It's not like American comics where it's being driven by companies and has rotating artists.