I just hit my 2 year anniversary of WordPress development. I’ve been a web designer for almost 15 years now. When I started my own business 2 years ago it was with the intention of doing regular static sites. Overwhelmingly my clients wanted a CMS so I dove into WordPress. For the first couple of months I felt the way you do. Once it “clicks” it makes all the sense in the world… I just encourage you to hang in there.
To answer your question though – the reason it doesn’t create a child theme by default is that just about anyone who wants to use the Twenty Ten theme does NOT know anything about HTML and CSS, they want to use the theme off the shelf, and they’re not going to do much customization and certainly not going to touch the code, and wouldn’t know how to create a child theme. Those who do modify Twenty Ten have probably been through quite a bit of the Codex (including the starter articles) in which it does indeed recommend the child theme.
Again, if you purchased a theme (or even used any of the other, non-default WordPress themes) they would NOT be overwritten by an upgrade to WordPress. That’s the beauty of how the themes are structured – the core files are not intermingled with the theme files. Twenty Ten (and Kubrick, formerly) ARE updated with a WP upgrade, because they’re “beginner” themes and WP is doing a favor by upgrading them.
Hope that helps… if you stick with it and keep learning more I think you’ll find that WP is indeed very user-friendly — for me, far more so than the “other” major CMSes out there.
Let me also add – WP is phenominal about always working to make it more user friendly with the new features they add with upgrades. Particularly starting with 3.0 they’re implementing CMS-necessary features (as opposed to blogging functionality only). Another reason you’ll definitely want to upgrade.
Good luck… keep an open mind!