Western society today is far more hygienic, kids spend a lot more time looking at screens of one type or another and probably more time indoors, buildings are better-insulated, kids as a whole are far more chaperoned by their parents, there’s more sugar in our food, less fat in our meat, and more processed snack foods available.
Any or all or none of these things could have a bearing on autism development. The problem is that if autism incidence is truly going up, any change in society over that period will be correlated with a rise in autism. So any amateur speculation or sham study will show correlation even if they have nothing to do with one another.
Quite true. And since we don't have reliable statistics on autism incidence, or even a fixed definition for the condition, all statistical analysis is bogus from the get-go.