Why do I need to clean flash between different Android versions on custom ROMs but not on OEM updates?

When flashing custom ROMs, I've noticed that it's recommended to perform a clean flash (i.e., wipe the device's data, cache, and system partitions) when upgrading between different Android versions (e.g., from 12 to 13). However, this isn't the case for OEM updates, where I can seamlessly update from one version to another without performing a clean flash. What are the technical reasons behind this difference? Why is a clean flash required for custom ROMs but not for OEM updates?