Because it is part of a manufacturing process that tries to save every penny because that translates into bottom line profit. These companies don't manufacture these things en mass, someone else does for them. Though they have control over it, the QC is generally done by these contract manufacturers and as such, may not always be perfect. I've been in the industry for many years and I have worked with the very same CMs that do this for Sony and Microsoft. The 360s are/were built just a few miles from my house for years, as were the PS2s. I don't think they do the PS3 here though.