Because young adults, by and large, are naïve, gullible and idealistic, and vulnerable to influence and molding.
Unfortunately, the LGBTQ cult has been very good at PR, propaganda and indoctrination, particularly in the public and university school systems. Combined with the world's natural hatred of Christ, the results are sadly not surprising.
Christian parents will send their impressionable children to a public school to be catechized in secularism for 7 hours a day, 5 days a week, for 12 years or so, and then further catechized at a typical woke college for 4 more years or so, and are then are shocked when their kids depart from the faith.
In short, Christian parents need to stop delivering their offspring to the Devil on a silver platter and need to engage in homeschooling and other alternatives.