They don’t have a power ground to prevent hums. There might be a marginal risk if something comes loose in the amp, but your modern electrical cabinet would have a differential switch to save you in that case. I remember my audio engineer degrounding one of my audio components when in had the issue.

My guess for the audio ground is that older turntables had poorer electrical isolation and power lines were more noisy. For more technical explanations we need an old school audio engineer.

Antenna ground needs to be used for signal to be read properly.

Turntable audio grounding (different from electrical power grounding) may help prevent hums when the common 0V grounding passes by the power ground.