Adjectives are declined in Polish, just like other languages. So because this is a non-masculine nominative plural case the ending changes to an "e".
Don't get too hung up on it though; you'll get a feel for what to do with adjective endings as you become more familiar with the language.
Take a look here if you want to know more about this particular case:
Polish has two genders in plural: 1. masculine-personal and 2. not-masculine-personal. So if you are talking about men you use "duzi": chłopcy-boys, mężczyźni-men, żołnierze-soldiers. If you are talking about other people, animals or things you use "duże": dziewczynki-girls, kobiety-women, stoły-tables, koty-cats.
Thank you so much! This should definitely be in the Tips & Notes. It's an interesting difference with Eastern Slavic languages which have no gender distinction in plural adjectives.
Don't mean to sound rude with the following question but, what is the point of genders? Could the Polish language decide to use male and get rid of female and neutral, what type of confusion would that bring?
Not every grammatical feature has a semantic function. You could argue that irregular verbs is a totally redundant feature of English and say haved, writed, thinked... from now on. But would you ever think of that?