I'm thanking Wizard69 for Post *8; an excellent description of general machining parameters and threshold faced by DIY to accomplish it by milling. Terrific!

Taking some liberties now with machine tool engineering, physics, mechanics, with a bit of 'apples and oranges'.........

There's a reason lathes were the first machine tools; and historical samples emphasize hurdles faced developing a mechanically simpler machine. There are complexities in milling completely different than turning. However, one set of elements they share with mills, is two-fold. 1. Spindle rigidity/ distance between bearings and 2. Maintaining that alignment to cutting axis.
Lathes and drill presses direct force along the spindle axis, mills introduce additional loads across it.

Compare resistance offered by mass [width & depth] of a lathe bed (12" machine for example) to it's cross-slide, against column necessary for a mill to do same (say a Bridgeport pattern = 12" X axis).

Next might envision a heavy Monarch or American Pacemaker turning along a cylinder 1/2" deep; few operators attempt facing off very same material and DOC (depth of cut).
Compare that to size mill required to push a decent 1" or 2" endmill same 1/2" deep.