"Are you sleeping?"
Well, because it does. The Russian grammar requires it. What explanation did expect to hear when asking that question?
I can't tell you much about the etymology, because different verb forms for вы and ты is likely to be a very old common Indo-European feature, and its origin is lost in the ages.
I works like this in most languages (e.g. compare Portuguese «tu dormes?» and «você dorme?», German »schläfst du?« and »schlafen Sie?«).
In Russian, вы is grammatically plural. When you address someone politely, you use plural form as if you were talking to several people. And вы requires a different verb form.
Also, every verb have 6 conjugations. The normal form "to sleep" is спать, the conjugations: Я сплю, мы спим, ты спишь, вы спите, он/она спит, они спят. You could split them up to three singular and three plural, I just like to think of it like the chart I learned in class.
Firstly, сейчас means "now".
Secondly, the correct form of the verb with ты is спишь.
If you want to ask "Are you sleeping now?", the Russian would be Ты сейчас спишь? However, we are translating "Are you sleeping?" which is simply Ты спишь? (Note: the corresponding questions using вы are Вы сейчас спите? and Вы спите?)