Why do I need a hardware wallet?

I see this advice a lot here and I'm curious...

Tell my why I need to throw $60 USD at a hardware wallet. What is so horribly insecure about:

• Using a LUKS encrypted USB or other medium to store private keys under a strong passphrase.
• Only accessing the device (and therefore putting the ethereum private key into RAM) on a secure computer that will only run up-to-date software provided directly by a mainstream distribution (e.g. Debian, Fedora)
• Suspending LUKS and stopping ethereum software when the computer is unattended
• Not using any ethereum software except for Parity and go-ethereum
• Using common sense, an ad blocker, and HTTPS everywhere