... because you think WordPress doesn't scale horizontally? That's... look, you should probably ask your doctor about WordPress.com. Or any reasonably large WordPress site.
Scaling web servers is easy but there are limits on what a single database instance can support that Wordpress won't address and having multiple sites on multiple database instances is a simple, desirable solution.
WordPress works great sharded and/or replicated across databases, servers, even datacenters. This is what WordPress.com, WordPress.org, and pretty much any major WordPress site uses: https://wordpress.org/plugins/hyperdb/.