Why phones with Android One don't get update to Q right now (2019)?
The updates are still required to be done by the OEM. And if you try flashing a GSI yourself, it will either not boot or be ridden with issues.
The underlying vendor partition also needs an update along with new and updated drivers and HALs from the component vendors to make the hardware work properly with the new OS version.
Hence, it takes time to merge, develop, test and then make the final market release. I wish updates were as easy as picking the Google system image and sending it to the phones. GSIs normally work ok if you're installing the version for which the installed vendor partition was built. But in my case they weren't fully smooth. For example, my Nokia 7 has a badly performing modem when on a GSI. And the Android 10 GSI just kept on bootlooping.