Why don't they use 500 bills anymore?

Answer 1

Printing of large-denomination bills was discontinued in 1945 due to low demand - at that time the dollar's purchasing power was many dozens of times what it is today so there was very little need even for $100 bills, let alone $500 or $1000. (FWIW, the bills were printed up to 1945, but the last ones issued were under the existing 1934 series so they all carry that date.)

Today there are 2 reasons that it's unlikely new high-value bills will be printed.

First, the use of credit cards and electronic payments for large purchases means there still isn't much demand for high-value bills, even though inflation has caused prices to rise to a point where $100 bills are commonplace.

Second, the government is worried that new high-value bills would make it easier for criminals to transfer large amounts of money without leaving a trail. With the $100 bill as the highest denomination available, smugglers or drug runners who want to move large amounts of money are forced to handle huge quantities of paper making it more difficult for them to avoid detection.

However, the EU does issue €200 and €500 bills, so there is some speculation that the U.S. may eventually need to follow suit regardless.