Because it's actually fairly slow.
Among other things, the OSD was not designed with NVMe drives in mind - which is fair, given how old it is - so it's nowhere close to being able to handle modern NVMe IO throughput and IOPS.
For that you need zero-copy, RDMA etc.
Note that there is a next-generation OSD project called Crimson [0], however it's been a while, and I'm not sure how well it's going. It's based on the awesome Seastar framework [1], backing ScyllaDB.
Achieving such performance would also require many changes to the client (RDMA, etc).
Something like Weka [2] has a much better design for this kind of performance.