It's not just the young people in the Netherlands who don't know any indigenous folk, it's been going on much longer.
Even my grandfather doesn't know any Dutch folksongs (well, a few songs for children maybe, but that's it). So, if you want to play Dutch folk, you'd have to refer to old songbooks (and the material in there can be very old, going back to the late Middle Ages). And learning music from books is not very appealing.
Luckily, there has been a small revival in Flanders, check out the young group Laïs, for example.
Anyway, the result of this is that when I joined a folk group at the local music school when I was about 16, we mainly played Scandinavian,
Eastern European and a little Irish music. Which I love, but nevertheless: there are many young 'folkies' here, but they just don't have the opportunity to learn Dutch folk from someone.