Why did 405 look so good?

[Mike Watterson]

Extract from the RCA book "TELEVISION The Future of the New Art
And In Recent Technical Developments" 1936

From page 343:   Scanning requirements

360 scanning lines  excellent
480 scanning lines  equivalent for practical purposes

This comparison assumes advanced stages of development for each of the line structures.
We may say therefore that a number of scanning lines in the immediate vicinity of 360, say 340 will give a very good performance comparable with 16mm home movie film.

So it follows that the 405 line system with it's 377 active lines can be considered as satisfactory.

They were indeed comparing with cheap news grade 16mm, or cheap duplication of 16mm for the home market and picked 441 lines, as you can't just have any number due to limitations of the timebase generation.
Similarly the H & V pixel count is best an integer multiple of 16 for digital systems.
Interlace is evil for  Digital or conversions or upscaling, however it's irresistible x2 compression for "free" on any analogue system as resolution perception is poorer as frame rate goes up and static or slow pans look nearly twice as good as simply having 1/2 the lines.

so interlaced
360 = 180
480 = 240
405 is 377 = 188.5 line

Hence CGA IBM was 640 x 200 graphic mode to allow for overscan on an NTSC TV.
EGA and VGA needed progressive scan screens.