Future Trunks and Dr. Gero would be appalled to see how far Androids 17 and 18 have come from their introduction in Dragon Ball Z and their continued presence in Dragon Ball Super. The twin sibling cyborgs are both parents now -- a far cry from their villainous introduction as the ultimate weapons of mass destruction. So far removed from their days of simply being known as Dr. Gero's creations, it's odd that they still go by their numeric designations rather than their actual names. After all, Androids 17 and 18 were born as normal human beings.
Almost certainly because of their eyes, the twins were named after a blue gemstone, Lapis and Lazuli. Not much is known about their lives prior to being turned into androids, but they were abducted by Dr. Gero as he searched for new subjects for his experiments. After turning the pair of them into androids, Lapis was designated as Android 17 and Lazuli as Android 18. Stripping them of their birth names would make sense for Dr. Gero's purposes, as it would further dehumanize them and serve as a reminder that they belonged to him.
The dehumanization definitely worked, considering that Android 17 immediately killed Dr. Gero once he'd awakened the twins. After they personally introduced each of the Z Fighters to the ground, the duo went on to roam the land, doing whatever they wanted as they searched for Goku so they could kill him for funsies. Interestingly, one thing they didn't do was go back to their human names, instead referring to each other as 17 and 18, even when in private post-Gero's death. It would be more than understandable if they wanted to distance themselves from the monster who kidnapped, tortured and changed them, but just killing him was apparently enough for them.
In Dragon Ball Super, over a decade later, they still go by and answer to 17 and 18. Considering they're both heroes and parents now, it's possible they've kept their numeric designations as a form of penance. They didn’t devastate the planet as they did in Future Trunks' timeline, but they did enough in this timeline that they still have blood on their hands. Keeping their names from their days of violence and cruelty might be a way of taking responsibility for their crimes rather than running from the past by changing their names.
Perhaps they've even kept the names in an effort to redeem themselves, so that "Androids 17 and 18" can be spoken of with love and respect rather than fear and hatred. If so, they've done a stellar job, as they've both fought for the good of the planet on multiple occasions. Android 17 even saved the entire Dragon Ball multiverse by winning the Tournament of Power and using his wish to restore the universes that had been destroyed. 17 also works as a park ranger now, protecting beautiful and endangered animals from poachers, even willing to give up his life for them.
The most likely explanation for why they have kept their names from Dr. Gero is that they just don't consider themselves to be Lapis and Lazuli anymore. Their lives as humans would have been so different from their lives as androids that they may as well be different people. Living in the past has never helped anyone, and their past as normal human beings is something they can never go back to. After all, even Shenron could not turn them back into humans. For better or worse, "Androids 17 and 18" is who they are now -- and to their credit, they've decided to own it.
