If I were to wildly speculate, I might guess that many Americans think of gender as being about presentation and experience, while "race" is almost entirely about shared experience - which, yes, is often informed by reactions to one's appearance, or lack thereof, but that's not the identifying characteristic.
So it comes across as similarly distasteful to someone claiming to be "long lost Uncle Eddie", because they saw your family through the glass at a holiday and liked what they saw - since you weren't here for a billion shared little experiences, and there's no claim that you were from the same grandparents or similar, it rings hollow and like you want something from it.
Interesting idea. How much shared experience is there though - for example between poor and affluent people of the same race? What about immigrant groups who look similar but have very different culture and experiences? I cannot imagine an affluent black African immigrant will have much in common with a poor black American. I know that (in the UK, but it would be just as true in the US) that I do not have much shared experience with other people who look like me (South Asian to Western eyes) but come from different cultures types of area (urban vs rural is a big difference) and family backgrounds.
So it comes across as similarly distasteful to someone claiming to be "long lost Uncle Eddie", because they saw your family through the glass at a holiday and liked what they saw - since you weren't here for a billion shared little experiences, and there's no claim that you were from the same grandparents or similar, it rings hollow and like you want something from it.