That wouldn't help. Without wanting to go too deep into OT-territory:
Even if the DPI could be known (and afaik normally it isn't, it is just set to a value) and it would be possible to convert in into real inches (btw, I have no idea how big that is in real units) to be displayed on a monitor, how would a designer know how big any element, even text, should be? It depends on the available space, which depends on the size of the monitor/the size of the window.
To think about how absurd that would be, try to find a situation where someone would say "this heading should be 2 cm tall".
px as size lead to the same problem with the available space, but they at least correspond to the resolution.
Even if the DPI could be known (and afaik normally it isn't, it is just set to a value) and it would be possible to convert in into real inches (btw, I have no idea how big that is in real units) to be displayed on a monitor, how would a designer know how big any element, even text, should be? It depends on the available space, which depends on the size of the monitor/the size of the window.
To think about how absurd that would be, try to find a situation where someone would say "this heading should be 2 cm tall".
px as size lead to the same problem with the available space, but they at least correspond to the resolution.