AT&T does offer symmetrical speeds. Most PON-based FTTH ISPs I've encountered do.

There are technical reasons for offering asymmetrical speeds when it comes to coax connections. There's only so many channels on the wire, a lot are still dedicated to television, people mostly only care about download, so they prioritize it. PONs don't carry television and are just fundamentally different in design, so its easier for them to support symmetrical bandwidth.