usql is a great tool if you're familiar with Postgres' psql client and wish you could use it for other databases like MySQL, Cassandra etc.
dbcli tools are designed to preserve the usage semantics of the existing tools but improve on them by providing auto-completion. For instance you can use `\d` in pgcli and `SHOW TABLES` in mycli. This was a conscious decision to make pgcli and mycli drop in replacements for of MySQL and psql. I was also working under the assumption that people rarely use multiple databases, you're either a postgres shop or a MySQL shop. If you have a mix of both, there is a good chance that not a single person is interacting with both of them on a daily basis. You have different teams using different databases. But my reasoning there could be flawed.
There is nothing stopping someone from adding an adapter to the usql tool to make it behave like MySQL (because they like the mysql client better) based on a command line argument, for instance.