There are two reasons for this: wood has some inherent advantages, and the alternatives in the market today are not ideal.
Wood is a strong material that can endure some extremely heavy loads without cracking or warping. When treated, stored, and maintained properly, wood pallets can also resist the test of time quite well, often lasting decades without the need to be replaced.
Finally, wood is a renewable material that is relatively easy to work with. On top of that, wooden pallets have been the standard for so long that there is plenty of infrastructure around to build those pallets on the cheap. There are also entire companies dedicated to helping businesses get pallets in Portsmouth fast when needed, as this website showcases.
Most of the wood’s advantages cannot be found in metal or plastic pallets, which tend to be much more expensive, on top of having some other disadvantages when compared to wood. Metals are too heavy, and plastic is too weak.