Why is the dollar bill so cheap?

If you mean cheap in purchasing power, it's due to inflation over long periods of time. As prices have risen the amount each dollar (or pound, euro, yen, etc.) can buy becomes less. There are lots of causes for inflation and it would take pages to explain at even a high level.

If you mean why is it such a low denomination bill, it's because the US is pretty much the only industrialized country that hasn't replaced its $1 and $2 bills (or their equivalents) with coins. There are a lot of reasons for that - Americans' resistance to changing anything familiar, a lot of politics, lobbying by Crane Paper, and so on come to mind.