> There is no reason to suspect that this would be useful for human nutrition

Do spiders have some kind of different macronutrients?

I don't know about macronutrients, but the required micronutrients would be very different. For example, spiders have hemocyanin instead of hemoglobin in their blood, so their oxygen transport is based on copper instead of iron.

(Maybe not the best example because cow milk is not a good source of iron either, but I'm sure there are other more relevant differences like this.)