It's just a different mindset. Not necessarily a _sane_ one, though...
Wouter insists that Ruby Gems are enough of an argument to keep Rails at a distance. Even though I agree with the basic claim and think that Gems are basically insane and sick, this should be taken a bit more under perspective.
We are blessed. We are blessed to have Debian, such a rich OS with such a great package management system, and with superb integration between so many packages. Blessed are also the users of most Free Software based distributions, as they share the advantages of systems growing with full consciousness of their interaction's benefits. However, integration with the rest of the world is not seamless.
Most scripting languages have their own infrastructure for managing the modules/libraries/pacakges/whatevers dependencies. Perl has CPAN, PHP has PEAR, Ruby has Gems... I do see Gems as the most obnoxious of them all, but the basics are the same.
The Rails crowd started being Unix-centric, but the Windows (and MacOS - they are no better, believe me, at least in this regard) world has exerted its pressure. Gems caters very well to their needs, but we do suffer the integration at the distro side.
The only sane way the propietary-minded people have managed to stay clear of the well known "DLL hell" is to ship everything a given program requires bundled together - that's the main reason for the bloat of lots of applications, and for the sloppiness of security support. Every application packager is responsible for shipping updated versions to any library it bundles in, except for the very basic core that the OS itself provides. That seems so annoyingly backwards to us that... it is unbelievable.
So, yes, Rails application trees often include Rails itself. For $DEITY sake, even I have grown used to working that way, as things tend to break under your nose otherwise. My proposal (which we talked over at DebConf, but have not pushed so far) is to support simultaneous versions of Rails installed in a Debian system (of course, via different packages), more or less in the way that simultaneous versions of Ruby, PHP or Python (and, in some limited fashion, Perl - Although Perl does not suffer from this incompatible bumps. Yay for Perl!) can be installed.
...And, yes, together with the pkg-ruby-extras team, I have been trying to -slowly, yes- package whatever modules we often use so they don't have to be included in Rails applications.
So far, the best way (although by far not optimal) I have found to limit this explosion of trees is to include most libraries in my Rails application trees as git submodule trees - If they are not explicitly downloaded, the systemwide libraries will be used.
Yes, the "ship the whole thing as a bundle" approach is quite annoying. However, at least I must acknowledge that it works better than the approach I took with previous (mod_perl-based) webapps I wrote... As relatively few people grok mod_perl, I ended up with quite some apps which were not installed by anybody but me. Rails is obnoxious... but seems to be parsable by the humanity at large.