The way you process/modify metrics vs logs vs traces are usually sufficiently different that there's not much point in having a unified event model if you're going to need a bunch of conditions to separate and process them differently. Of course, you can still use only one source (logs or events) and derive the other 2 from that, though that rarely scales well.

Plus, the backends that you can use to store/visualize the data usually are optimized for specific signals anyways.