Just recently started trading options and certain aspects seem to good to be true. After looking over theta curves  to decide which options pay me the most with the least risk I have concluded that 6 week covered calls appear to be the best. for me.
Now for the questions,  I used RIO to do my calculations since I own shares of that but I checked a few others with similar results.  By selling options that expire in 6 weeks it seems that I come up with an annual return of upwards of 20% solely off covered call profits. 

If the call ever gets exercised the profits would double or even triple according to my math. 
I know there's a flaw here somewhere or everyone would be doing this.  Can you explain some of the variables that would limit  my gains or cause losses using covered calls?
Hi Tim,
Keep in mind - when things appear that way, they are almost always are too good to be true.  But covered call writing is an excellent strategy. To work the way you are hoping, several things must be true:
  • If you sell a call option in RIO that expires in 6-8 weeks
  • If you keep selling new options when the old call expires worthless
  • If the call option you sell is out of the money by about 3 points (as it is currently - assuming you are looking at May 35 calls)
  • If the options continue to trade at the same price levels (sometimes they are higher, sometimes they are lower)
Then yes - you can expect to collect about $6 over the course of one year.
Your math is correct - the profits would be much higher if you were assigned an exercise notice on the calls you wrote.  You would (in this example) be selling stock at 35. But there's a bit more to it, as outlined below.
Sure - here are some of the variables  There are reasons why 'everyone' is not doing this.
First, not everyone bothers to understand how options work.  Also, many people never heard of options and the brokerage industry doesn't encourage people to get involved.
Second, many people are enthusiastically bullish.  Covered call writing limits your profits, as you would not be able to sell stock any higher than the strike price.  Some are not willing to sell their stock at the strike price (or any higher strike price).  They dream of hitting the jackpot.  They want their stock to go to 50, then 100, and then split 2 for 1 six times and go to 100 again.  In other words, they want to find stocks that rose like MSFT, or WMT.  They are just greedy and don't want to play the covered call writing game.  I think that's foolish, but to each his own. 
Third - stocks do go down.  If you write covered calls, you gain some protection.  Thus, you are better off than the stockholder who does not write calls.  But, you can lose significant amounts if the stock tumbles.  Thus, this strategy is bullish in nature and does not appeal to people who are less bullish. 
Next, this strategy is for people who plan to hold onto the stock for a period of time (at least until the option expires).  Thus, day traders and swing traders would not be interested in this strategy.  Covered call writers must have some patience.
In the real world, it will not work as you see it today.  for example - what would you do if the stock rises above 35 and you sell your stock (because you were assigned an exercise notice on the option you wrote)?  Would you buy back the stock and begin writing covered calls again?  Let's say the stock goes to 37.  Would you be comfortable at that level?  Would you be okay writing the call with a strike price of 40, or would you be too worried that the stock might go down in price?
Today, you are looking at the stock near 32.  What would you do if the stock were priced at 30.25 after the next expiration?  Would you sell the two-month 35 call for only 30 cents when you'd really like to get 90 cents?  If you decide not to sell, but to wait for the option to get to your 90 cent target, it might be many months before you ever sell an option again.  That would severely cut into your expected gains for the year.  On the other hand, if the stock is 34, you would get far more than 90 cents when selling the new call with a 35 strike price.  So there is some luck involved in just where the stock is priced after your option expires.
You want to sell 6-week options.  Well, that's not the real world either.  After expiration, you will be forced to choose between 4, 8, or 9 weeks.  Fours times per year, you will be able to sell an option that expires in 5 weeks.  Note - if you like theta, then you don't want to wait.  You want to sell your call option as soon as possible after expiration.  Just letting you know that 6 weeks is not going to be available to you if you chosose to sell new options as soon as you can.
Some stocks are much less volatile than RIO and their options are priced much lower.  If you deal with volatile stocks, yes the rewards are outstanding - but so are the risks of a stock collapse.  Still, if you plan on owning these stocks anyway, then I am 100% in agreement - write those covered calls and collect that premium.  Give up on the chance to make an overnight killing in the market and the odds of coming out ahead of the person who simply owns the stock are very much on your side.
All in all, this is a good strategy, and I use it for my portfolio.
Best of luck.