It is because of the technology that is used. Real D uses polarized glasses that work with the type of projection used in theatres. Home 3D uses active shutter glasses to create the 3D effect (which is why home 3D glasses are so expensive). The type of projection used in theatres is very expensive, but allows them to use cheap glasses - home 3D uses (relatively) cheap technology in their displays, but advanced technology in the glasses that you must use with the television to get the 3D image.
As for your second question, no I do not know of any web sites that show anything in 3D. My prediction is that 3D technology is not going to make it anyway... at least not until they can produce a display that does not require you to wear expensive glasses to view anything in 3D on it. They have been trying to push out 3D technology since the 1950's, maybe they should be working harder on hologram technology.
For 3d to work they take a camera that has 2 lenses spaced apart like your eyes and each lens records a complete 2d movie. They record what you would see with your 2 eyes if in that same spot. Then they just show both 2d movies on the screen at the same time. Because of the lens spacing on the camera without the glasses most images don't match up and you get that double vision effect. Now your right eye can only see the right lens movie and your left eye the left lens movie, so they use glasses and can now put that movie into different 3d formats with different kinds of glasses whose lenses can block each movie in different ways.
For polarized 3d at the theater the projector passes the right eye movie through a filter to give it one polarized light wave, and the left eye movie through a filter to give it a different polarized light wave. Then each lens only lets one light wave through. Your tv or monitor was not made to put out 2 different light waves, it just puts out random ones. Now they can make polarized tv's. They have to put a coating on the screen so all even numbered horizontal lines of pixels put out one light wave and all lines in between those the other light wave. Then each movie is sent to the correct line. This is called row interleave, column interleave uses vertical lines instead. Acer has a row interleave polarized 3d laptop computer. The 5738DG.
Then there is field or frame sequential 3d. This is where they flash between movies left-right-left-right-left-right-etc. very fast in sequence, hence the name. Then the glasses open and close left-right-left-right-left-etc. so each eye sees it's own 2d movie. Some movie theaters use it, most tv's and computers use it. Radio signal tells lenses when to open. For this format you tv or monitor must have inputs and be made to switch between 2 movies.
Only anaglyph 3d works on all color displays. They passes the righ eye movie through a red filter to give it a red tint and the left eye movie through a cyan(blue-green) filter to color it cyan. Then you have red and cyan glasses who's lenses only let one movie through. If green and magenta is used in the 3d movie, you have to have green and magenta glasses.
Also, to get the 2d version of Avatar, they just show the right eye movie or the left eye movie by itself.
Below is a youtube video with a 3d tab, put cursor on it and you will see the 3d formats I mentioned, and more. The reason for row interleave 1 and 2. 1 has the right eye movie sent to even numbered horizontal lines and 2 it goes to the odd numbered lines, depending how your display is set up. There are lots of videos on youtube that use the 3d tab.
https://www.youtube.com/watch?v=822kH5Fu8SM
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