The Sources of all hair extensions are either natural, or synthetic. Natural ones come from natural human hair. This is far more expensive than synthetic, because it is in short supply. Someone had to grow their hair out, cut it, package it up for distribution, and provide it to a hair extension manufacturer or seller for resale. This makes it much more valuable than its synthetic counterpart, which can be produced on demand for a wide variety of synthetic fibers.
Years ago, the huge diffrence in quality between natural human hair and hair that was made in a factory was readily apparent; synthetic versions simply did not look "real", even to the untrained eye.
Technology has changed over the years though, and hair extensions are no exception. Now, synthetic hair extensions are indistinguishable from real hair to all but the most trained eye. Quality has risen, while pricing has fallen. Now, sales of synthetic hair vastly exceed sales of natural hair for use in extensions.