Why are domestic wire transfers free in France but cost ~20 USD in the United States?
Because US banks would rather use (free) ACH instead.
Delivery of ACH transfers can take several business days, meaning days that banks are open. Unlike the real-time processing that wire transfers have, ACH network operators process transfers in batches only three times a day. […]
ACH debit transfers, including payroll direct deposits and most bill payments, are typically free. If you need expedited bill payments, there can be fees. For ACH credit transfers, banks might charge a fee of around $3 for sending money between accounts that you have at different banks, but many offer these so-called external funds transfers for free. There’s usually no fee to receive them."
A wire is a real-time method of transferring immediate funds and supporting information between two financial institutions and is relatively expensive to use. An ACH is similar to a wire transfer only it uses a batch-process. Transactions received by the bank are processed in batches and funds are not available in the beneficiary account until the next business day. ACH transfers are less expensive than wire transfers.