[fckgw]

So in the case of the iMac, you have the "chin" on the bottom, which is where all the computing guts are stored. The new M-series iMacs are incredibly thin and the only thing above the chin is screen. There is also a webcam on the top which takes up a bit of space so they make the sides the same width to match. If you also look at how the LCD is inlaid into the chassis, they need to have a lip around the frame in order to hold it in. This is going to require a bezel of some sort

https://www.ifixit.com/Teardown/iMac+M1+24-Inch+Teardown/142...

The screen on the Studio Display is pretty similar, it doesn't set "into" the frame like some monitors, it sits on top of the chassis with a small lip. It has a camera (and basically an iPhone) in it as well but the bezel could probably be shrunk down a bit.

I wouldn't doubt that they made a design choice to keep the bezels similar between the two products to make a cohesive design language. If/when they shrink bezels on one expect the other to change as well.