I was reading about excess and in one of the examples I found this,
They both eat to excess.
Why didn't this use 'too' instead of 'to'? It does not make sense to me, I don't know why.
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"To" is correct because "excess" is a noun.
Here, "to" is a preposition that indicates the extent. This is from Merriam-Webster:
d (2) —used as a function word to indicate the limit of extent
// stripped to the waist
In your example sentence, it means they ate so much that it was excessive.
"Too" is incorrect because it can only modify gradable words, like certain adjectives and adverbs. "Too" is sometimes used before a noun, but it means the noun is being used as an adjective or adverb:
This painting is beautiful, but too Warhol for my living room.
In this example, "Warhol" is understood to mean "Warhol-like" or "similar to Warhol's art".