system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why don't ATMs give out $10 bills

It frustrates me to no end.
gpt
I have seen plenty that do. Its very much dependent on the bank/location. 

But the actual answer to your question, is that most atms only have one drum, the container that holds the bills. So only one type of bill can be in an atm. $20 is big enough to do some spending, but not too impractical, most places can break one no problem.