Why are scrambled eggs in the plural?

The plural is preferred: the google reports 233K results from "scrambled eggs are" and about 61K from "scrambled eggs is." From Wikipedia

Scrambled eggs is a dish made from whites and yolks of eggs ....

From "How to Make Scrambled Eggs":

Scrambled eggs are an easy and popular dish for breakfast.

As an adjunct noun, the same lopsided preference obtains, "scrambled eggs recipe," "over scrambled egg recipe."

I guess it depends on how hungry you are when you're writing about scrambled eggs.

It's one egg, two eggs, no matter how you serve them. From Spoonful of Promises: Stories & Recipes from a Well-Tempered Table by T. Susan Chang:

At length, I found myself with two egg variations that Noah would eat. Inevitably, they each took a half hour to make (yep, that's thirty times the amount of time it takes to make a scrambled egg).

It doesn't matter that they don't keep the form of eggs. They were "eggs" before they were scrambled, and the term "scrambled eggs" refers to the result of scrambling (multiple) eggs. Grammatical number doesn't have to correspond to the physical form of something: the phrase "a doughnut cut into fourths" is grammatically singular even though it refers to four pieces.

You can't say "one scrambled eggs," just as you can't say "one eggs." You can say "[one/a] scrambled egg" if you started out with one egg, and scrambled it. If you started out with multiple eggs, scrambled them, and want to refer to a single piece of the result, you can say "a serving of scrambled eggs" or "a portion of scrambled eggs."

Oxford Online Dictionary defines "scrambled egg" and "scrambled eggs" as a mass noun:

[MASS NOUN] 1 (also scrambled eggs) A dish of eggs prepared by beating them with a little liquid and then cooking and stirring gently.

I don't think it has anything to do with a number of eggs put into a plate/dish of scrambled egg(s). They are just one of many kinds of food that we eat. That's why it could be classified as a mass noun.

Mass noun is:

In linguistics, a mass noun, uncountable noun, or non-count noun is a noun with the syntactic property that any quantity of it is treated as an undifferentiated unit, rather than as something with discrete subsets. Non-count nouns are distinguished from count nouns.

[Wikipedia]

"Egg" is a countable noun. No matter how many eggs you put into scrambled egg(s), it becomes an uncountable noun. You have to use "a plate of" or "a dish of", "two plates of", etc. to count it.

The below Ngram Viewer shows no usage of "a plate/dish of scramble egg" and "a portion of scrambled eggs". "A plate of scrambled eggs" is much more often used than "a dish of scrambled eggs".

Note: There is always a question whether you could use "two scrambled eggs" in place of "two plates of scrambled egg(s). You could use the former, but it might be confusing because it could mean either "two plates of scrambled egg(s)" or "scrambled egg(s) with two eggs". It is clearer to use "two plates of scrambled egg(s) or "scrambled egg(s) with two eggs". Ngram Viewer shows "two scrambled eggs" is broadly used.