I have wondered the same thing my entire career of drafting. People refer to ANSI and ISO standard without actually nailing them down as no company ever want's to pay for said standards.
I have had to create drafting standard at three companies.
When pen plotters first came out in the late 80's and AutoCAD's lineweights were Pen mapped, I use 0.35mm and 0.75mm for D sized drawings. and used 0.35mm and 0.25 mm for B sized drawings.
With 600+ DPI printers and A/B sized drawings (I only use A) as the norm, I picked 0.25mm for a balance of crispness and visibility. I have considered 0.22mm a few times, but never went there. Anything smaller than 0.18 makes drawing hard to see. Anything larger than 0.25 makes drawing a blur if you are showing many hidden lines in a view.
I use that weight for everything, but use 75% gray for anything other than object lines to help separates the centerlines, hidden lines etc...
The font size is paramount. A slight change in it's size really effect the available space and readability. To best balance that I use the following font.
3/32" Tacoma! And I don't allow any smaller subscripts etc. EVERY piece of text and dimensions on the page are 3/32". Tacoma allows one to tell the difference between and i/1 and o/0
Any smaller, people under age 40 can't reel without glasses. Welders need the largest font you can give them.
Any larger, you don't have enough room to fit things on the page.
From the ASME standards, ASME Y14.2 M, Line Conventions and Lettering, section 2.1 Line Widths:
Two widths of lines are recommended for use on drawings (see Fig. 1). The ratio of line widths should be approximately two-to-one. It is recommended the thin line width be 0.3 mm minimum, and the thick line width be 0.6 mm minimum. All lines of the same type shall be uniform throughout the drawing. One width of line is acceptable on drawings prepared by a CAD (Computer Aided Design) system.
I have attached the two relevant pages from the standard.
I have the 14.2 standard and after just manually reviewing the SE line weights it appears SE is using the .3 and .6 thin and thick concept.
Most drafting standard were created when drafting was manual, and D sized prints were the norm. How many small companies have the ability to copy 24X36 drawings any more?
More comapneis than not will use standards for creating CAD files that are D sized, but then print them on B size making the text much to small to read and cut the line weights in half.
I happen to work with A size only. If I used 0.6mm lines, Most of my drawings would be a blur.