A downloader-installer is pretty pointless and doesn't allow offline installs so makes it not appropriate for everyones use.
If people really do want installers, one possibility is to do like there is for GIMP, there is a GIMP for Windows project on Sourceforge and you can download a Windows installer from there. This would mean no extra package to be carried by dl.xonotic.org. A Xonotic for Windows and Xonotic for Mac project could each be started by people who want to do such a thing and each time a new Xonotic is released, they just need to rebuild their installer and update on Sourceforge. I would suggest if someone particularly wants to do this they should look at how projects like Libreoffice/OpenOffice.org, GIMP, 7-Zip etc. package for these propreitary systems. I wouldn't want to maintain such a project myself but I would perhaps be interested in helping make an initial install for Windows on Sourceforge just so that there is something which is technically workable for someone else to carry on with. Mac I know nothing about so can't help.
As for Linux distros, it's the same thing again that Mr Bougo says, it's down to the distros to carry packages so all volunteer to your distro to be the official maintainer if you want Xonotic in it.
I'm at least a reasonably tolerable person to be around - Narcopic