User can change browser for any repository or AUR package after first boot as part of first update
Do you expect Calamares to offer more?
If a browser is installed from a Live ISO it could be a few months out of date so it should be updated before use
Better solutions for more choices would be a netinstall or manual installation
Xfce minimal ISOs did not include Firefox until earlier this month. 20 days later, this topic suggested changing default browser. 75% of votes so far are to keep Firefox
Anyone who wants a choice of browsers in Calamares could always post a feature request
But poll suggests only 18 voters so far are interested in ‘something else’. Not a popular bandwagon
Manjaro is funded by user donations and sponsors and does not seem to be lacking funds