Search also sucks because people suck.
If I post a picture of a flower with the caption "Look what grew in my garden!", that's a terrible post from a search point of view.
Later on someone will search for "flower" but I didn't use the word "flower" so now search sucks.
Of course a much more common post is someone posting a picture of text, from Twitter, Tumblr, etc. with, once again, a vague caption. You remember the picture, but not what the poster actually said.
Searching comments will sometimes help, but that depends on the comments being related.