This sounds like a flippant response, but it's true. But more importantly, did you know that some of these plugins work by simply clicking
activate while others require several steps.
My favorite "can't live without" plugins require placement of tags within the template files that I need to set parameters for to get the output I want, and I need to place them where I want them to show up. Where would you like some of these to show up? In the sidebar, header, footer, or only when viewing a single page or only on the front page or in the archives or categories?
Some of the plugins that require tags to be placed within the template files will crash the site if the plugin is deactivated. I have a bunch of notes to remind me to activate them after upgrading and making other more sophisticated changes to my site, which is a lot to remember.
Plugin technology for WordPress is still evolving, like the software, and they are becoming easier and easier to install as authors get more educated and aggressive about making their installation easier. But some still require the user to "do" something to make the plugin work where and how they want it to work.
I think adding plugins to themes is a neat idea, but only if it doesn't make the user's life more miserable than it would if they were to pick and choose from among the hundreds of neat stuff that is out there. If I like a theme and it comes with the weather icon plugin, I'll have to figure out how to uninstall that since that is information I don't want on my site.
It's a tough choice, but a good question. Someday it will be easier to say "turn on this plugin, place the tag in the footer so it shows up on the right below the Apple Category link and next to the Orange Icon and make it visible only when viewing category pages", but we're not there yet.