Not always true. Some character just have very similar powers. Miles and Gwen, although alternate universe variations who now live in the same universe, are usually younger and less set and would do with a mentor, like Peter who has been their and done that but maybe wants or needs to take care of other things.
Legacy characters, with powers, tend to be harder to do, But given that in Comics alternate characters getting similar powers from alternate sources is far from difficult, Spider-Man being a legacy character is not unreasonable. Remember "Spider Island" and all the other spider-men who got their powers in differing ways, spider spirits genetic alteration, mutation, blood transfusions, gadgets and technology, and so on? But there has to be at least a good set up for it. I don't think Peter Parker could make Spider-man a legacy if not for "Spider-Verse" or "Secret Wars 3"
Some super-powered characters might be able to be come Legacy characters. If they had not made Franklin Richards a veritable God, and Valeria being raised by Dr Doom, all while the Richards Family is falling apart, like a sit-com family would in real life, I could see the Fantastic Four becoming legacy though their children and the Future Foundation.
So, I do disagree that Super-powered characters cannot be legacy. Just not all of them and not always easily and certainly not the exact same way. While Miles and Gwen have many of the same powers and Peter they are not the same and they don't share the same mental and/or emotional attributes. Franklin and Valeria Richards would have been very different from their parents filling in their shoes.