Here are the top three reasons why.
3. They cheat.
The number of crossword writers who own a dictionary that's less than five hundred years old is the same as the number who know what partial synonyms are: zero. But even if you are the kind of person who knows what "oast" and "uie" mean and also thinks "pony" is a synonym for "racehorse" there are still ways that crossword writers can cheat. Foreign languages, abbreviations, and puns are the bane of crossword solvers everywhere, but by far the most insidious trick is compass directions.
By an unspoken rule all words in a crossword are required to be at least three letters. For compass directions this means you'll be looking for one of the off-point directions, for example ESE (east-southeast) or NNW (north-northwest). There are a total of eight possibilities and these never give anything more than "compass direction" (or occasionally "wind direction") for their clue, meaning they basically translate to "pick three letters at random and know you have a one in eight chance of being right."
2. They don't make you smart.
Quick! What's a four letter word for a pitcher? Don't bother looking it up, because every regular crossword solver here already knows it. For those who don't, the answer is "ewer." I don't know that because I'm a genius, I know that because it appears in crosswords all the time.
When you do crosswords regularly you don't really learn a whole lot, you just memorize the words crossword writers love to use. For example, if you see the word "Egypt" in the clue all you need to do is check the number of letters available and write down either "asp," "Nile," or "Cairo" depending on how many you have, and that's not the only thing that works that way. As far as crossword writers are concerned, Panama and Erie are the only canals, Thor is the only Norse god, and asters are the only type of flower.
1. No, seriously, they cheat like nothing else.
Alright, now that you've warmed up by probably not guessing the word ewer, it's time for another. This time it's 21 letters and the only clue is "DICK." That one appeared in last Sunday's L.A. Times. If you're curious, the answer was "Jefferson's Mockingbird." Who on Earth could actually guess that? Who WOULD guess that? The word "DICK" could mean so many different things, especially if you're giving me an entire 21 letters for the answer. (For example "guy who wrote this puzzle" is exactly 21 letters.)
Crossword writers could put a five year old playing monopoly to shame. They use archaic spellings (or even incorrect spellings) for normal words whenever possible, they ask for only part of words, on at least one occasion they had me rearrange another word into gibberish, and their idea of a clue is "it's a word, good luck."
All of this, of course, means that you need to train yourself to recognize clues, just like I mentioned in the last point, because guessing based only on your own intellect is impossible. In other words, being intelligent and thinking outside the box actually makes you worse at crosswords, while being dumb and easily trained makes you better. Keep that in mind the next time your aunt who smells like cats brags about how she can solve a crossword puzzle in under five minutes.