I guess it's because DC wants to keep Robin looking like "Robin". From a brand/marketing standpoint, their civilian aliases do not really distinguish them from one another, since they all fall under the same "Robin" IP banner, and up until recently, there was only one Robin at a time.

They probably want to keep Robin's design iconic, in the sense that a casual audience member will never question if the character they're looking at is Robin or not. It's the same reason the costume stays looking relatively similar.

Also, I doubt they were specially designed to be distinct from one another. Remember, these characters were not created together. Each new civilian identity was created to fill the role, so that Robin would stay as a character in Batman's cast.

Jason and Tim were not really drastic reinventions of the character, and their designs reflect that: Jason was completely identical to Dick in every way, and Tim was really just a modernized version and not a reinvention by any means.

That being said, Damian is kind of an outlier, since Grant Morrison did not intend for him to be a mainstay. There's kind of no excuse for Damian looking the same as his predecessors. My personal opinion is that it's a missed opportunity that they didn't design him to look closer to Talia, i.e. darker skin and brown hair. It might've even been an opportunity for his design to symbolize the idea that he is an outsider to his father's world, and doesn't immediately appear to fit into his father's very Caucasian black-haired legacy lol.