Why are network devices typically accessed through a CLI rather than a GUI?

Because a GUI can fail, crash, or hide options, while the CLI is lightweight, scriptable, consistent, and works even over slow links — so it is the reliable way to manage network devices.

A GUI is friendlier and needs less knowledge of the underlying command structure, but it depends on more resources and can fail, crash, or simply not expose everything a device can do. The CLI, by contrast:

• Is lightweight and runs even on low-resource devices
• Works reliably over slow or unstable remote links
• Is scriptable and gives consistent, repeatable results
• Exposes the full command set with direct access to the configuration

For these reasons, network devices are typically managed through the CLI.

Go deeper:

• Jeremy's IT Lab — Intro to the CLI (CCNA Day 4) — a hands-on tour of the Cisco IOS command line and why network engineers live in it.
• Command-line interface (Wikipedia) — why text commands are scriptable, repeatable and lightweight compared to a GUI.