Because most people aren't as thorough as you are. You will also see that most people list her as being born in Lexington, which is a feat unto itself, being as Lexington wasn't even founded until 1712. Up to that point it was Cambridge Farms.
One thing I have learned with this Online Genealogy is that way too many people accept information as fact just because they found it "online".
I was burned badly once and had to go back and undo all the information I had found once I realized that it was all based on speculation, poor speculation at that. At yet another time I was contacted by a researcher who was convinced that a certain Fiske was her female Fiske's father just because he was the only other one she found in the same town at the same time. When I checked my data I asked if she had seen any birth records for him, when she gave it to me it confirmed what I had, he was only 8 years older than she was. I know that some people married young but really...? I asked her how she missed that fact and she said she hadn't even noticed.
Back to Elizabeth, I believe that Elizabeth was the child of Lydia as well for the same reasons that you list. She also had to be born before Nov. 29th, 1654 because that is when Lydia (Cooper) Fiske died.
After 300 years I don't think there is any physical proof that Lydia was the daughter of Lydia Cooper and David Fiske because by now it would have made it's way into some records data base or repository somewhere.
Good luck, Bob Birt
P.S. I am also descended from this line. Elizabeth's older brother David is where my branch seperates from yours. (my list is as follows: My mother-Helen Lucy Fiske, Benjamin, Benjamin, Benjamin, Benjamin, Benjamin, Ebenezer, David, Lt. David....)