Why don't Marty's parents ever mention how similar he looks to the Marty of 1955?

It doesn't matter

His parents never mention the similarity in the movie because:

1. Before the 1955 part of the movie, it would be somewhat of a spoiler.




2. After the 1955 part of the movie, it might have worked, but it would have been "cute dialog of convenience" and not really realistic there.

Ultimately, if you accept the underlying premise of Back to the Future which is that if key events stay more or less the same, the future will be unchanged, then you have to accept that whether his parents saw the similarity or not is irrelevant. This is good news. You can either choose to believe they never noticed a similarity, or you can choose to believe that at some point in Marty's alternate youth, his parents showed him a picture of this guy he bears an uncanny resemblance to, and none of them including alternate past Marty had any idea it actually was him yet, and then they went about the rest of their lives normally. Whether either theory is correct is irrelevant, because according to the rules of the BTTF universe, it doesn't actually change anything.