Yeah, I wondered about that too. I remember older, traditional window managers like TWM, FVWM, OpenBOX, etc. show this but most newer "Desktop" environment's window managers like XFCE's and Gnome's do not.

Interestingly when sizing a terminal window these WM's show the size in columns and rows and when sizing a "real" window it shows the size in pixels.

Probably an option somewhere to turn it on in newer environments but it's been a while since I used anything but cwm[1] (which does show the window size on resize) so I can't be sure.

[1] calm window manager - https://en.wikipedia.org/wiki/Cwm_%28window_manager%29