Not sure why you believe that is the case.
I suppose it really depends on what you consider to be "young".
Most teenagers and 20 somethings are in school and being an "author" isn't exactly a career that makes a young person's heart skip a beat. It's not up there with being an entertainer or athlete.
Self-publishing requires a financial & time investment on the part of the author to get the book published as well as promote it if she or he is (serious) about earning some money.
A lot of young people aren't even interested in working for anyone let alone building their own business or becoming an authorpreneur. You have to be a hustler to have a business.
Lastly a lot of young folks don't feel they have anything compelling to say. Unless they grew up around people making money as authors or were encouraged to be writers most students would not consider writing as being a viable career option. Achieving dreams requires inspiration, motivation, and determination. Writing is considered boring to a lot of folks.