So you're shifting the terms of engagement from "want to" to "willing to"? There's a difference...
And let's just say, as to the crux of your point, your experience with adult readers and mine clearly differs. Kids are "willing" to read classics (if they don't "want" to) when and if there's something on the line (ie, grades). Adults are "willing" to read classics largely when there's something on the line, too -- they feel they should, the book has been in the media or on Oprah (and they feel societal pressure to participate, etc).
Are there adults who read the classics for pleasure? Sure.
And there are kids who read the classics for pleasure too.
But they get pleasure out of doing so -- ie, they find it fun, and it interests them. Adults and kids alike.