Event details
Because you can target policies to different devices/users which may, when combined, cause conflcts. Intune is not aware of a "complete" deployment for a single device so cannot validate and check if any conflicts.
Up to you to resolve conflicts.
Even happens between built-in MS baseline policies.
But ...it does become aware right? After the fact, 2 days later, when the device reports back in? Seems like a solvable problem.
MS used to have a conflict fix feature "Policy Health Workflow" which was part of Windows AutoPatch, but was retired in Sept 2024.
You can get a report of conflicts across polices in the Devices > Monitor > Assignment failures report.
I think the functionality you mention about checking before you hit save, is the bit that would not be easy, or even possible, to implement.