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Why would you begin to think that at all?

Because they're offline doesn't mean that they're safe, just safe from more than an online system.

Those systems might be immune to attacks that networked machines are vulnerable to, but often networked attacks are used in order to leverage another exploit, such as Meltdown/Spectre.

A vector other than the network could be exploited to leverage an attack that utilizes Meltdown/Spectre.

Most the biggest software vulnerability catastrophies actually involve just such systems. "Secure internal network" is a 90s mirage. See eg https://www.wired.com/story/notpetya-cyberattack-ukraine-rus...

Meltdown & co. only become relevant once you can execute on a CPU. If an adversary can run software on your industrial robot, you are already compromised.

Think browsers and virtualization, and the various VPN's that invariably connect "internal networks" to the outside world via computers that straddle, or alternate between, other networks.