...because they're XML. Browsers wish they could handle XML but they don't.
We are looking at https://ifarchive.org/indexes/if-archive/games/aas/ here.
There's some discussion of XML files at iftechfoundation/ifarchive-ifmap-py#27 . I have taken the position that we should let the browser try, but this is a good argument against that.
Another option is to serve .aas as application/octet-stream (in apache-conf-ifarchive.conf).