They accidentally fixed them when they changed the terrain generator in Beta 1.8. The only way they generated at all was because of an accident in the first place.
EDIT: In response to Phycozz, the invisible wall was added later to prevent people from going to the buggy "fake" chunks that generated where the far lands used to be.
Even the current 30 million block limit is artificial; you can check the actual code yourself (search for 30000000; this is for 1.6.4, before they added a wall, explaining how "fake chunks" happen; the code simply returns 0 instead of a block ID, etc, after that point); there is no viable explanation for such a limit in terms of what variables can handle; a 32 bit integer can range from +/- 2.147 billion, so they added that limit probably just to have a hard limit on world size - not that anybody would ever have a problem with it.
Also, I suspect they removed the original Far Lands by going from single-precision to double-precision variables; the accuracy of single-precision is close to the 12.5 million block limit; double-precision has far more accuracy, enabling a vastly larger world size (millions of times) before the math goofs up or integers overflow*. Of note, some things still goof up because they use single-precision variables, such as the position of torch flames (in 1.6 anyway), but when I tested it I didn't get any of the jittering present in the Far Lands or Bust videos (your position uses double-precision now).
The hard limit where chunks are overwritten is at X/Z of ±34,359,738,368, which is about 23% of the distance from the Earth to the Sun. At X/Z of ±2,147,483,648 (crashes at 2,147,483,439), item positions, mob pathfinding and other things using 32-bit integers will overflow and act strangely, usually resulting in Minecraft crashing.
DANCE IT OUT! WOOOOO
EDIT: In response to Phycozz, the invisible wall was added later to prevent people from going to the buggy "fake" chunks that generated where the far lands used to be.
Also, I suspect they removed the original Far Lands by going from single-precision to double-precision variables; the accuracy of single-precision is close to the 12.5 million block limit; double-precision has far more accuracy, enabling a vastly larger world size (millions of times) before the math goofs up or integers overflow*. Of note, some things still goof up because they use single-precision variables, such as the position of torch flames (in 1.6 anyway), but when I tested it I didn't get any of the jittering present in the Far Lands or Bust videos (your position uses double-precision now).
*From the Wiki:
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?
Kudos.