Of course it's got a resolution before it's scanned. Film stock behaves differently from pixels but there is a practical limit to the available resolution set by the size of the grain. Standard 16mm also uses a ridiculously small frame and therefore has lower resolution, as it simply has fewer available grains, but the image steadiness is probably as big a factor in the relative lack of quality.
One should also keep in mind that TV cinematographers would light their scenes very quickly, often using too much light because the TV sets at the time didn't handle darker tones very well, and because quite a few sets were still black and white.
Not to mention the fact that the scanners were sometimes of fairly poor quality.
And yes, most TV shows were indeed shot in, or at least composed for, 4:3 until the late 80s or early 90s, even when using 35mm cameras (in which case they would commonly use something called "full frame" and compose the image for both the 1.33 TV format and the 1.85 aspect ratio in theatres). Spielberg's "Duel" is an example--TV movies of the week were often shot with 1.85 in mind in the hope that they could be released in cinemas overseas.
And yes--old films often look crappy in Blu-Ray. One reason is that sometimes the negatives are in bad shape (or were crappy to begin with, which is often the case with really old films where the film stock was often of very poor quality), another that a crappy TV scanned print (or even some old tape) was used, a third that some old DVD or tape scan has simply been upscaled to 1080p. They wouldn't have to be, most of the time, but that would require a proper rescan of the original master negative.
Edit: Aren't we all glad that we started this discussion?
