Why? Because notifications are not shown by default. The watch vibrates but nothing is there on the screen but the watch face. To trigger a notification to actually show, you need to rotate your wrist down (if it is up) and then up again (impossible if you are carrying something). Then you have to wait for it to recognize the gesture. Then you have to wait for a fade animation to an intermediate screen that shows half of the information about the notification. Then you have to wait for a fade animation to the final screen that actually shows the notification text. And if you lower your wrist at all during or after this process, the notification disappears instantly and won't come back without the use of both hands to activate the touchscreen or crown. And in many cases the information shown is useless, like "so-and-so sent a picture." Show me the goddamn picture then!
It boggles the mind that anyone could think this is a good experience to ship at all, let alone for three generations! Especially when Pebble had it right 10 years ago. You show the notification on the screen first, then you vibrate the watch, then you leave the notification there for at least 30 seconds so it can be read. You show the whole notification at once (as much as possible on the small screen, of course), and you also show a small clock at the top so you can still see the time while the notification is shown. How hard is that?
Of course this isn't the only way the Pixel Watch software sucks. The whole UX is pretty terrible, like they had some ideas and went straight to final implementation, never bothering to iterate based on user feedback. Low information density, low customizability, feature-poor, unresponsive interactions, blocking animations. It's everything bad about modern UI in a tiny frustrating package.
I don't know what is wrong with all the tech reviewers that fail to call Google out on this stuff. Maybe it's better than Samsung or whatever but that doesn't make it good. At least the hardware is nice. Though, like all smartwatches except the Pebble Time Round, far too thick.