Android has an easy remote backup system built in. You can save a file to any location, including cloud locations, as long as the cloud service provider plugs into the API. Signal actively disables this feature because they would rather spite users than risk even the shadow of a chance that a user upload an encrytped backup to an internet service that could theoretically then be hacked and hypothetically maybe one day decrypted.

Matrix doesn’t have this issue, it just stores encrypted messages on servers.