Why is 'the' before winter season?

Rather than seeing the definite article in this usage as indicating a particular winter, I see it as (at least usually) referencing the particular season that is winter.

The hunting season. The holiday season (is when most hoteliers make their money).

The diesel locomotive (can mean 'as opposed to the horse, the electric 'locomotive', the steam locomotive – ie a particular type of motive power – or 'the actual piece of metal I'm talking about').

The floribunda rose (was introduced by the Danish breeder Dines Poulsen in 1907).

This is not a question of semantics, but of syntax.

The word season does not fall in any of the classes of noun which can be used without an determiner (plurals, mass nouns, proper nouns) and so requires a determiner (an article, a demonstrative, a possessive, a quantifier etc). Having a modifier such as Winter before it makes no difference.

Winter itself is different, having some of the characteristics of a proper noun (not all of them: it may take the, whereas typical proper nouns don't).