Is there any obvious reason why commodity ETFs such as GLD, SLV, USO never pay dividends?
Is it because the commodity market attracts a lot of speculator/traders/hedgers and not exactly long term investors?
No, it is not for the reason you suspect. Some long-term investors might actually allocate a small percentage of a diversified portfolio to a commodity ETF, as a hedge.
Rather, the obvious reason (and it really should be obvious!) is that a hunk of metal or a barrel of oil don't by themselves generate any cash flow. There simply is nothing to distribute.
Worse yet—it actually costs money to store and maintain physical commodities somewhere. Even if the commodity ETF is synthetic and uses derivatives, and no physical storage is required, there are bankers with mouths to feed.
If you are interested in dividends, I suggest you look at a novel legal structure known as a "corporation". Many corporations are adept at generating profits, and some even distribute a portion of their profits.
There are even corporations who specialize in extracting commodities from the ground, if you want to still have exposure to hunks of metal, etc. while getting some yield. Bear in mind that a company that extracts something from the ground isn't the same thing as the thing from the ground. They won't necessarily move together.