Very ignorant take. If all of us had infinite infrastructure there would indeed be no reason to separate the three at generation, but for those that have to actually count the costs, paying for servers to store that amount of duplication is real issue, and even more (for some reason, by order of magnitude or more) if you're using one of big observability providers.

It is divided mostly because database to store metrics, and database to store essentially freeform documents are very different products and need different features from query language, and having "just metrics" allows to store them with compression reaching 1byte per metric data point or less.

Logs and traces I admit, split here is unnecessary, and it should be just presentation difference. But metrics is often just unrelated to any single event summary of the state of the system. Sure you can just log every single log entry with request metadata and get request rate, percentiles etc. by calculating the events, but that is just too wasteful for most.