The leading theory for the cause is (and bear in mind I’m simplifying a heckton here) “we don’t know”. There’s possibly a genetic component involved, but if so, then it’s super complex and doesn’t have a clear causal mechanism (in that a person could have most/all of the genes of interest for autism, and not be autistic, and we don’t really know why that is. Or why some people have none of those markers, but still have autism). Environmental contamination is a possibility, but we don’t know nearly enough to guess what that would be, given that it wouldn’t necessarily be microplastics

But there definitely is a lot of evidence supporting OP’s claim, in that we are very confident that our increased understanding of autism has led to an increase in diagnoses (especially amongst groups such as girls; I am one of those girls who only got a diagnosis because of our shifts in understanding). We are extremely confident that our rate of false negatives has dropped over the years (i.e. people who actually do have autism but go undiagnosed). However, it’s hard to estimate whether the actual underlying true rate of autism has actually gone up, given how much our understanding has changed in a relatively short span