No you are not really missing something. Ethernet is a LAN interface (generally) and as such IOS will not allow 2 interfaces to be configured out of the same subnet but serial interfaces are treated as point to point WAN interfaces.
I know that's not a complete explanation - i'm afraid it's just one of those quirks in IOS.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...