What are named something like “limited liability company” in other countries are, as GP said, not equivalent to LLCs, they are approximately the same thing as US corporations.
US LLCs are a comparatively newer business form that was created mainly to be a streamlined form (compared to a corporation) for corporate joint ventures. They've since become popular for other uses beyond the original focus.
What do you mean? LLC aren’t AFAIK about tax: they are about limited liability. How is that different from other countries’ LLC-equivalents?