It’s because you’re watching HD scans of shows that were never intended to be projected at the full resolution of their 35mm camera negatives. Of course you’d be seeing a shitload of imperfections.It's a good thing, I mean it as a compliment, but I guess it also describes an on-screen look that's perhaps a bit eerily perfect - again I'd bet it's down to modern cameras, filters, and makeup artistry, which make already-attractive people look absolutely flawless, whereas older cameras seemed to always show skin imperfections, sweat, and dodgy makeup up much more sharply, and often in way harsher lighting.
All the actors look great IRL of course.