Why are there so many people allergic to all nuts yet we don’t see many people allergic to all fruits?
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Not a lot of people are allergic to all nuts. There are just several different types of nut allergies that it is hard for manufacturers to keep track. Especially since a lot of processing is with mixed nuts. So it is easier to avoid any nuts whatsoever then to try to find out which nut products you can eat and which ones you can not.
There are exceptions though. A lot of peanuts tends to be processed separately due to the amount of it we eat. So someone with a nut allergy that does not involve peanuts might still eat some mass produced peanut products like peanut butter as it is pretty certain there is no other nuts mixed into it. But for example a chocolate covered peanut could have other nuts added to the chocolate to enhance the nut flavour. We also see the same with almond which is another mass produced nut. Seeing almond on a cake might suggest other nuts have been used as well but marzipan on the cake is generally safe because only almond is used to make it and it tends to be processed separately from other nuts.
The antigens in nuts might look more like each other than the ones in fruits do. Some of these also look somewhat like certain pollen antigens. Plus as another person mentioned, it might also be down to non-biological factors such as how food manufacturing processes may be separated.
In actuality, though, it’s not so much that these antigens from different kinds of foods resemble *each other*. It’s that they all have some features of antigens found in [helminths, which our IgE class of antibodies mainly evolved to target](https://www.reddit.com/r/explainlikeimfive/comments/140edg9/eli5_allergies_why_are_there_certain_foods_eg/jmvhm41/).
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