Maybe it does and no one told Google?
Wireless & Location
Wi-Fi 2.4GHz + 5GHz 802.11 a/b/g/n/ac 2x2 MIMO
Bluetooth® 5.0 + LE (HD codecs: AptX, AptX HD)
The Pixel 3a and 3a XL do not currently support AX WiFi. Whether that can be added in a future update, only Google knows, but currently it doesn't and more than likely never will.