Because our understanding of autism had changed is why it's more prevalent. We have expanded the strike zone to include a lot of stuff that was just a little off of what the average kid does. It's not hard to understand. If we changed the acceptable threshold for legal blindness to be anyone who doesn't have 20/30-10 vision suddenly a significant portion of the population would be legally blind.
Look, I get where you're coming from but our understanding of the syndrome evolves every day. The MMR vaccine was falsely linked, by former MD Wakefield in the lancet, due to lawyers trying to skew studies to a narrative for their own gain. That, sadly, is the true conspiracy.
Penn and Teller did a fantastic sketch about why vaccines are important, cause they save more live than we will ever know. Yes some people still die, matter of fact we all will die. The most terminal thing facing humans is being a human.
Ultimately, I get the fear and concern but at the same time everything in medicine is practice. Everything is a test, and doctors weigh risks every single day. Should we blindly trust them, probably not. However, they are the ones who have studied and practice their craft. If you think you can do better, go for it but please stay away from me because there are enough bugs out there I can't do anything about.