US silver dollars dated 1935 or before are silver, some special collectors coins for the Eisenhower series were struck in 40% silver but only from 1971 to 1976 and were not released for circulation.
Because silver is far too expensive. Coins used to contain silver until about 1920 in the UK, 1965 in the U.S., and 1968 in Canada. Since then they are made from a mixture of copper and nickel, so...