https://www.cnet.com/news/google-pixel-3-doesnt-support-qi-fast-charge/

We don't limit third-party devices, in fact, we're working with our partners in the Made for Google program to get fast 10W chargers certified for use with Pixel 3 (Belkin announced its 10W Pixel 3 charger already which will launch in the coming weeks). Pixel Stand and Pixel 3 work together through the protocol we've developed for fast charging. Everything else charges at the industry standard Qi 5W.

Google hasn't indicated why it took this approach, but it's possible it's linked to the extra features that are activated when you pop the Pixel 3 on a Pixel Stand.