That's a really good question Monika. I thought about it, and wanted to see if you could do the same thing with other question words (who, what, where, when, why, how), and in fact, you can't. It only works with "why". But you can use a bare infinitive (without "to") to ask things like: "Why eat pasta when we could have steak." "Why study German if you're going to France." "Why lie to my brother, when I could tell him the truth." "Why live in Switzerland, when he likes Italy so much more." I'll guess you'll just have to chalk it up to one of those things you have to learn, without having a good reason!! :-) Alan. (PS: the fact that you even ask this question means that you're observant and tune in to language anomalies well!)

According to Michael Swan, Practical English Usage (an excellent reference book) we use Why + bare infinitive to suggest an action is unnecessary or pointless. Why bother, why worry and the sort of examples Alan gives.

"Why bother?" is pretty unique grammar in English and may be confusing to understand, but I think if you hear it used enough you will understand its use. I will give you some examples. This will be the pattern -> "Why bother...[doing a pointless activity]. [reason the activity is pointless]...anyways. "Why bother taking the kids to the museum? They aren't going to enjoy it anyways." This sentence is saying that taking the kids to the museum would be a waste of time because they won't enjoy it. "Why bother talking to that girl? She is going to ignore you anyways." "Why bother fill that bucket up? It has a hole in it, so it will all leak out anyways." "Why bother going to work today? You're going to get fired anyways." This pattern is used often enough that it is worth your time to practice it. Good luck!