Balls - the thing is the way pele talk of their experience around things like only children always seems to be heavily laced with prejudice and discrimination that doesn't work the other way arund. On answer to the OP could legitimately be, imo, "because I couldn't bear the non-stop negative comments about only-children'.
In all truth, I think that there probably are some generalisations that are more true of only-children than others - and that this is true for all kinds of families. I do make allowances - like I make sure that i enable DS to make good friendships - I welcome other children into our house, and I do lttle self-audits to make sure I am not over-indulging him because his staus makes it possible. But all parents do this. they make sure that a middle one doesn't get ignored, or that the only boy, or girl, doesn't get to be king/queen of the roost, or isolated. etc etc.
BUT although I hear a lot of parents of two or more children making disparaging cmments about single children and attributing it to being an only, when do I ever feel the need to say 'oh, my friends children irritate me, the conmstant whining and arguing between them, the fact that my friend has to keep interrupting the conversation to sort out disputes, it's because they (loud sigh) have more than one child'. I don't . Kids are kids and we don't need to generalise about any of them. But parents of only children constantly find that every little thing about thier child is attributed, critically, to the fact that they happen to be in a family without siblings.
I know, I know - whatever you do as a parent you get someone making a comment...it would be nice if this one was one that could be laid to rest, amongst the others.