[squarefoot]

> Why can’t either of these systems do what the Mac has been able to do since the 90s, and display the recursive size of a directory in bytes in the file manager

Many file managers can do that, although for obvious reasons it's rather built as a contextual action on a single directory than an always on feature than would slow down the filesystem horribly by accessing it recursively on many levels. On Thunar (XFCE's file manager) for example it's accessible from the contextual menu opened using the right mouse button on a directory name; other file managers would work in a similar way.

I'm sure filesystems could be modified so that any write would automatically update a field referred by the containing directory, so it would quickly propagate to the upper level, but that would imply many more write accesses which for example on SSD media would do more harm than good.

[fennecfoxy]

Mac doesn't for me, for a folder it shows size as "--".

[voltaireodactyl]

Open display settings (CMD + J) and tick “calculate all sizes”. May take a second if you have some huge directories.