I think quoting oneself is like talking about oneself in the third person... it's socially awkward.
Also the basic format of a quotation
"words words words" - attribution
is like using bold/italics/underline all at once... conveying to the reader that you think this point is super critical/insightful. If you walked up to someone and said "listen to what I'm about to say because it's really insightful", I suspect it'd provoke the same reaction as quoting yourself in writing does.
Unfortunately, if you just write the one line you want to say, without acting like it's a quote at all, you have to rely on someone else to add the bold/italics/underline and pass it on. Also, it'd really stand out on your quotes page, not being in quotation marks and all. ;)
It's socially acceptable for other people to excerpt individual sentences from things you write, but not to do this oneself.
It's interesting to think about why. Probably because it's so hard to get a useful idea into a single sentence that even trying to seems presumptuous.