Many do, and even large DBs can be accomodated by throwing terabytes of RAM at the problem, but etcd is fundamentally targeted at a different problem domain than a general-purpose database. Everything in etcd is represented by a single-threaded state machine, which is great for simplicity and correctness, but that comes with performance limitations.
https://etcd.io/docs/v3.3.12/dev-guide/limit/ https://github.com/etcd-io/etcd/blob/master/Documentation/op...
TiKV, a distributed kv store that can store many terabytes of data, actually uses etcd internally for metadata storage.