"The girls are pretty."
Translation:Ta děvčata jsou hezká.
It would be "Ty holky jsou hezké," because holka is feminine. Děvčáta gets ta and hezká because it is grammatically neuter, even though it refers to a feminine person. There is a lot of useful information about gender and case at this site: https://en.wikipedia.org/wiki/Czech_declension.
Because that's how it is. Really, grammatical genders are mostly random. Why is člověk masculine and osoba feminine? It does not have much sense to ask these questions unless you want to make a review deep in the history of genders in Proto-Indo-European, Balto-Slavic and Proto-Slavic languages. And then how Old Czech preserved or changed these. Děvče may actually be a sort of a diminutive, which tend to be neuter.