Replacing "When" With "For / Since" In A Question?

Those alternatives do not work very well in that sentence.

"Since" and "for" are used in the same way as "because"; that is, the complement should describe a causal relationship and they should be used in a declarative statement.

You could say, for example, "I wonder why you keep buying plants, since you just end up killing them." In this case, I am making a declaration ("I wonder why") and the reason for the thing I'm declaring is the fact that you end up killing the plants.

In the statement you have, "when" is a subordinating conjunction which heads an adverbial clause that modifies "buying." You could use "if" in a similar role: "Why do you keep buying plants if you just end up killing them?"

In informal conversation, I would not be surprised to hear someone swap in one of the words as you have suggested, although the grammar does not hold up to scrutiny when written down. The declaration ("I wonder why") would be implicit in the question.

On second thought, I think “since” and “for” are literary while “when” is more vernacular. I think the fellow who answered you had a fair point! But in some areas one is allowed to consider exceptions. FIrst, there is clearly an implicit declaration, which is “I wonder”, in the sentence, and this brings me to my second point: Judging from the vibe this question gives off, one can very well consider it rhetorical. A rhetorical question is a way of proving a point. Therefore, “Why do you keep buying plants” can be very well interpreted as sarcasm, or as a declaration, instead of a sentence used to elicit a response. “You should stop buying plants!” In that case, “since” and “for” should both be permitted in such place: “Why do you keep buying plants since/for you just end up killing them.” We can sometimes be extra analytical and look for ways to break ground; this will bring a share of rewards in the future.