It's because nobody is that old, the original ones being issued near the end of the 18th century.
Just to be pedantic: the Thaler was first minted in Joachimsthal, Bohemia, in 1518. A Thaler is called a dollar in Spanish, and the Spanish version of this coin, minted in the Spanish empire's silver mining colonies in America, were the first American currency. A US dollar was a Spanish dollar of the same size and weight minted in the US much later - 1858 I think.I guess there might have been a few US ones minted before that, but the Spanish one was the "real" one for a very long time and was in main circulation. Arguable what is called a US dollar today is not, from the point when they went onto the gold standard with $5 quarter ounce gold coins, and the paper dollar since 1934. Dollars are supposed to be silver.
"It is better for civilization to be going down the drain than to be coming up it." -- Henry Allen