Because security and usability are inherently at odds, and Apple has always erred on the side of usability, until the security downsides are simply to great to

Because security and usability are inherently at odds, and Apple has always erred on the side of usability, until the security downsides are simply to great to ignore. This has been the pattern for every single security improvement in Mac OS X. If you understand the tradeoffs, you can do a wide variety of things to massively increase the inherent security of your Mac by changing system and app configurations.