I'd guess that some software is stuck on Power, so a bigger multi-core chip for those cases is probably cheaper than both porting to x86, or buying a higher tier Oracle license.
At least on Z series mainframes, LPARs are considered hard partitions (handled by PR/SM, not the OS like a KVM situation) and are actually treated as different machines.
I am not sure if this is the case with POWER.
I've never heard of this website and don't know if they are a decent resource, but they seem to cover the issue the way I remember it. Looks like they are offering a license management product that tries to warn Oracle customers about this consideration. I'm sure Oracle slaps people pretty hard over this during their first audits, maybe even hard enough to get a company to shell out for IBM hardware :).
https://bluemedora.com/the-hard-and-the-soft-of-oracle-licen...