It’s usually a mix of history, practicality, and image-quality limits rather than a hard timing problem.

Historically, long shutter speeds were limited by mechanical shutter timers, and 30s became a common standard. Modern cameras time exposures electronically, but that convention has largely remained because it works well for most users.

Technically, very long exposures bring other issues:

• Metering: camera meters often aren’t reliable in extremely low light, so automatic exposure beyond a certain point becomes less meaningful.
• Sensor heat and noise: on digital cameras, longer exposures can warm the sensor and increase thermal noise.
• Use case: exposures longer than 30s are relatively specialized, so Bulb mode is a simple way to handle them.

Bulb also gives the photographer direct control for uncommon long exposures and works well with remotes or intervalometers.

So the 30-second cap is mostly a practical design limit: long enough for most normal shooting, while anything beyond that is treated as a special case better handled with Bulb mode.