You are trying to understand mathematical expressions using French language grammar.   

"deux plus deux égal quatre" is a mathematical equation changed to words for ease of general communication, not a sentence. Just treat the use of the adjective as the name of the = sign as an “idiomatic” use, as an abbreviation of “le signe égal”.

If you want to translate from a mathematical equation into a grammatical sentence, use verbs égaler or faire.

Note: 2 plus 2 equals 4 in English (with English defining 'equals' as a noun, verb, link verb, or adjective to cover its varied uses), or either 2 plus 2 makes (faire) 4, or 2 plus 2 is equal to (égaler) 4.  Not that different from the French usages.

PS: égal is also a noun in French, but not defined for the use here in equations. 

https://www.dictionnaire-academie.fr/article/A9E0529

https://www.wordreference.com/fren/égal

Thanks but if "deux plus deux égal quatre" is a mathematical equation changed to words", then you are using égal as one of a sequence of nouns.. otherwise, it is still qualifying the expression, not describing it.

I am wondering if this is not simply a case of an abbreviation error in a very repetitive situation that has become accepted by habit

Mark - the equation is : 2 + 2 = 4. That is the language of maths. 

You are right, when expressing in words it is “word for” (numerical symbol) 2,   “word for” + (mathematical operation), “word for (numerical symbol) 2”, “ word for = (mathematical symbol)”, word for (numerical symbol) 4. Just as you cannot necessarily translate French directly to English, you cannot apply grammar rules of another language to the language of mathematics. Mathematical language doesn’t care about nouns, verbs or adjectives. 

To make this into a French sentence, use the verbs but to verbalise the mathematical expression, it is done in French as Aurelie has written. 

Ahh.. think I  see... égal can also be a noun which would fit what you are saying.. think I prefer the verb form (as I was taught) and hope I am not put in a situation where I don't have a choice

Mark - just remember when you speak the difference between égale and égal doesn’t matter, and if you are writing, it’s not the most egregious error either. Not in the same league as incorrectly using être froid(e) instead of avoir froid(e) !

Sometimes (often) it is better not to try to understand how language came to be, but just accept ‘it’s just the way it is’. 

Thanks Maarten.. suspect that, as much as the Institut might deny it, French, like English, is essentially a spoken language and the rules were created post hoc.. doubt if Shakespeare or Chaucer knew the difference between an adjective and an adverb... or Hugo for that matter!

Cheers 

Mark 

Agree !