I hated power bricks that came with laptops in the old days (i.e. ~15yrs ago). A 90W brick was huge and heavy to carry around. Today a 200W power brick weights less than that and is about the same volume. I think using external power supply for monitors is a reasonably sound design choice.
The body of monitor could be made thinner. For example, Dell's Ultrasharp series with built-in PSU, the depth (thickest region) is about 5.3cm. LG's Ultrafine series with external PSU, the depth (thickest region) is about 4.5cm. Thinner is better (..up to a certain point). Just aesthetically speaking, I find LG more pleasing to look at from the side.
Without a built-in PSU, the monitor's internal has less heat to dissipate. Easier and cheaper on a cooling solution. I believe Ultrafine series in recent years are cooled passively by well designed convection flow. For example, the
tear down of this 2018 model has not found any fans inside.
Less heat also means electronics going to last longer, operate more reliably and accurately (which is crucial for color). Touch wood. If your external power brick dies early, it's user replaceable. Very easy to find replacement because higher-power consumer electronic devices seem to be de facto on ~20V DC. That also means the power brick is re-usable for other compatible devices or even DIY projects if unfortunately your display panel/backlight dies early. It's even more environmentally friendly, so to speak.
Without a built-in PSU and perhaps cooling fans, leaving more room for other features. The same Dell series doesn't come with built-in speakers while the said LG series come with two 5W speakers built-in. I find it quite convenient for watching news clips. And my desk has one thing less (external speakers) to mess up.
Better yet, at least for me, the power brick can be hidden (by zip tie) under your desk. That is a much better place to install a PSU than inside the back of a display IMO
