Part of it is that the technology in the 70’s through the 90’s was nowhere near what we have today. The actors, their costumes and etcetera look better today than they did decades earlier.
Another consideration is that there is a continuity through part of the Marvel universe. Not only in look and feel (at least, among the Disney owned portions of the Marvel franchise), but with characters that cross over, then come together (i.e., The Avengers films).
To give you a better idea of what we are talking about… let’s look at the trailers below for earlier incarnations of the Captain America character.
Captain America (1990)
Captain America Double Feature (1979)
Captain America II: Death Too Soon (1979)