It’s a personal computer. Remember, the original PCs were “personal” because individuals could buy them and set them up for themselves. The Raspberry Pi is, if anything, a further continuation of that trend.
More than that, it keeps alive some of the original spirit that has gone from the Microsoft/Apple market: early PCs were hackable to some extent by users (remember when you got BIOS listings in the Technical Manuals?), but current proprietary machines effectively have a big sign across them saying “No User-Serviceable Parts Inside” (and this applies to both hardware and software).
Conversely, the Linux-running Raspberry Pi invites you to open it up and mess around, both in terms of hardware and software. And this is deliberate, by design.