Because جداً is an adverb, which means it describes how the action - or the whole sentence - is been done. Adverbs in Arabic don't agree with gender and number, they are just the same in every situation. A quick example with the main pronouns:

أنا تعبان جداً

أنتَ تعبان جداً

أنتِ تعبانة جداً

هو تعبان جداً

هي تعبانة جداً

نحن تعبانين (تعبانون) جداً

أنتم تعبانين (تعبانون) جداً

هم تعبانين (تعبانون) جداً

As you see, the adverb is always in the same form.