In the UK, there is Autscape. In the US, there once was Autreat, spearheaded by the incomparable Jim Sinclair. But Autreat is no more. This appears to be an unmet need at present.
(ASAN has an annual gala in DC, but nothing else AFAIK. An "annual gala" strikes me as approximately the most horrible idea since whenever high school prom was invented -- but to each their own, I guess.)
Why are there so few? I suspect that a conference that is truly by and for autistic people faces unique challenges, what with (1) the social and executive-function deficits that most of us share, and (2) the vast range of divergent characteristics that a conference would need to accommodate. I'm not sure how to meet those challenges, but much could probably be learned both from Autreat's remarkable ~18 years of success and its ultimate collapse.