Why are rain boots opened up on the top doesn't that defeat the purpose?

Rain boots are used to keep your feet dry while walking outside in the rain. Most shoes would absorb the water from the ground, and get your feet wet from the inside, however rain boots are meant to prevent that. They aren't meant to actually keep out the rain. However, most rain boots don't have a big opening, and it is hard for water to land perfectly into the opening. Rain boots normally have just enough room for your pants to slip inside, so even if water does get through, it won't touch your skin.

I hope this helped!