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.