I don't think I can give a definitive answer to your excellent question. However, I can point out that there are a few nouns in English which look and sound plural even when they are used as a singular word. When this happens, we sometimes treat the word as though it were plural – perhaps because it just sounds more natural to our ear.
Some examples would include:
These scissors are dull.
There is a hole in these pants! I can't wear them.
These tweezers are sharp! Be careful with them.
Similarly, some English nouns are plural, but they appear singular and therefore are treated as singular words – much to the consternation of some purists:
The data has been corrupted.
Note: some might argue that data is plural and therefore should be treated as a plural word, but there is an interesting blog post about that here. It says (among other things):
It's like agenda, a Latin plural that is now almost universally used as a singular. Technically the singular is datum/agendum, but we feel it sounds increasingly hyper-correct, old-fashioned and pompous to say "the data are".
An EnglishPlus grammar blog post entitled Tricky Plurals points out that branches of knowledge ending with -s (such as physics and ethics) are treated as singulars, as are diseases (such as measles and mumps), while words that end with -s and represent a pair (such as pliers or eyeglasses) are treated as plurals.
I find the word you ask about – looks – rather unique. It's no disease, it's not a branch of knowledge, and it doesn't represent a pair. Wiktionary defines looks with two noun definition:
n. Plural form of look.
n. One's appearance or attractiveness.
In that second usage, you are correct, we typically treat it as a plural noun, even though it's pretty much synonymous with beauty:
Her looks are stunning. Her beauty is stunning.
although we can use look in the singular, too:
She has an alluring look.