Recently, I asked whether a Crystal Ball could be used to cast Scrying many times by different people. The answer was that yes, I could, but it may take ten minutes of casting before another person can attune to it. But I'm wondering now whether the crystal ball must be held the whole time to use Scrying, a 10-minute spell.
The Crystal Ball allows me to "Cast the Scrying spell" while touching it. The long casting time rules say that you must "spend your action each turn" to keep casting a spell. I'd guess that, since I can't cast Scrying when I'm not touching the Crystal Ball, that means I fail on any turn I stop touching it, since I can't spend my action.
How does this work? If you can explain why this works with magic items in general, bonus points.