"Why do you say that?"

Translation: Proč to říkáš?

[Vlad_Gagaj]

Why isn't "Proč řikáš to?" right?

[VladaFu]

The second position rule for "to".

[HowardDavi16]

I am trying to understand the use of říkáte and říkáš. And in this case both are correct. Is the use of říkáte more formal?

[AgnusOinas]

"(ty) říkáš" - you (one person, friend or relative) say. middle English: "thou sayest"

"(vy) říkáte" - you (one person, stranger) say. OR you guys (y'all, multiple people) say.

[endless_sleeper]

říkáte is either plural or singular formal. I suggest either going through all those wonderful tips and notes sections provided (lightbulb icon, web only due to Duo being Duo) or consulting one of the recommended resources like Naughton's Essential Grammar.