Justifications for this:

0. TV hblank rate is 15,625Hz - at 50 fields/sec, that's 312.5 lines/field, some way short of the 400 you'd need to display 2 fields

1. Out of the 312.5 lines/field, only 288 are booked for the visible area, and most TVs can barely resolve them precisely anyway

2. When the TV is displaying one field in one set of scanlines, and the previous in the other, where does the data for the previous field come from?

2.1. TVs of the period have no buffer

2.2. Computers of the period don't generally have the RAM (or sometimes the hardware) to double buffer

2.3. The image produced is based on more than just the contents of the RAM anyway, increasing the RAM requirements (if you were to try to do this)

2.3. Cheap DRAM of the period doesn't have the bandwidth to scan out two frames'-worth at once)

Here's a pic of my BBC Micro, with interlaced output, running a program that flashes the screen alternating red and white: http://i.imgur.com/1XvkRso.jpg - you can see that at the top it's scanning out an entirely white frame, and at the bottom there's the end of the previous entirely red frame that's in the process of decaying.

As an example of 2.3 - note that the difference between one frame and the next here is entirely the video registers - the RAM stays exactly the same. Only one of the palette registers changes (and the flashing cursor is added by the hardware as a sort of post process step).

(I suspect the red/green/blue blur at the bottom of the white region is an artefact of my phone's terrible camera. Photos from my (slightly) better camera don't have that, but they do look overall the same. However today only my phone is willing to play ball with my PC.)