Think of the one word thuis as representing two words in English - "at home". It derives from te+huis. Te is a preposition that can mean at, to or in, but in this idiom it always means at, so thuis is always where you are, never where you're going. If you know any Latin, it is the locative case of domus (home,house) with a built-in preposition making a one-word statement domi (at home). You get the same idea in German with nach Hause, "to, towards home" (Dutch naar), and zu Hause, "at home" (Dutch te).