Arudino Uno has _always_ been primarily about being super simple for beginners. This is why the Arduino IDE exists (and why many more advanced hobbyists eschew it). This is also why the Arduino Uno can be used without breadboard. And why Arduino shields exist, so you can add on the Unos in plug-and-play kind of fashion, for beginners. I believe a secondary design goal is to still be useful past the beginner stage (which is where comparison with a Pico comes in), but that's not the _primary_ goal.
In contrast, Pico is far more complicated for beginners, since it's not designed for that purpose. You need a breadboard in order to use it with anything else. And the toolchain is _far_ more complex for beginners! Hell, I had some issues setting up the Pico toolchain on my Mac (and I'm not a beginner), whereas with the Arduino, it was just "get the Arduino IDE". Yes, you can use the Arduino IDE with a Pico now, but that wasn't that way from the beginning, and the fact that you can do so now is A) not because Raspberry Pi released any official method and B) because the Arduino IDE exists in the first place specifically to support Arduinos.
In short, as I said before, they are different products with different audiences, and therefore different design goals.
In contrast, Pico is far more complicated for beginners, since it's not designed for that purpose. You need a breadboard in order to use it with anything else. And the toolchain is _far_ more complex for beginners! Hell, I had some issues setting up the Pico toolchain on my Mac (and I'm not a beginner), whereas with the Arduino, it was just "get the Arduino IDE". Yes, you can use the Arduino IDE with a Pico now, but that wasn't that way from the beginning, and the fact that you can do so now is A) not because Raspberry Pi released any official method and B) because the Arduino IDE exists in the first place specifically to support Arduinos.
In short, as I said before, they are different products with different audiences, and therefore different design goals.