Why can't I use the definite article 'the' before winter? Isn't winter already in the accusative case, marking that it is a direct object?

"...in the winter" sounds better

Omitting the definitive article "the" leaves the English sentence hanging because winter can also be an adjective. The sentence would be properly complete by ensuring that "winter" is a noun with the addition of "the" or making it an adjective by adding "months"