libgconf-2.so.4 is no longer part of the LTS Ubuntu release. Running the launcher manually will give the following error:
./starmade-launcher: error while loading shared libraries: libgconf-2.so.4: cannot open shared object file: No such file or directory
The launcher binary hasn't been updated since 2017. You should build and test the launcher with each new LTS release (typically every years). A better option would be to drop the libgconf dependency, if possible, or use a Java-based launcher in the future that will work on all platforms.
Launching from steam doesn't have this problem for some reason, but launching directly does, whether it be from the steam directory, or from the launcher downloaded directly from the website.