Stipulating that you need a routing protocol here (you don't, right? You can do proxy ARP, or some more modern equivalent of proxy ARP.), there's a whole family of routing protocols optimized for this scenario, of which OSPF is the best-known.
Opinions vary whether this is a real concern, or just a way for the networking team to maintain their relevance.
All Calico cares about is that routes are distributed across various systems, they don't necessarily care how you do it (configure BIRD however you'd like).
BGP is surprisingly simple and easy to set up with BIRD. Setting up a route reflector with local hosts on the same L2 all peering with each other and suddenly you can route whatever IP's you want by announcing them to your peers.
Why do people think BGP is complicated?
This isn't complicated, it's config management. You can ignore 99% of what BGP can do in this use case.
Read your own paragraph before this question. Why do I need to run another process to exchange routes and configure a mesh or a route reflector? As an admin that's just another mess of processes and communication to worry about.
Just because BGP is easy for you does not mean it's easy for most server admins and devs without heavy networking backgrounds.
> As an admin that's just another mess of processes and communication to worry about.
Yet we fully expect admins to understand and build HA redundant clusters for databases, or how to manage and update all the machines under their control, and a variety of other tasks.
There is nothing inherently different about running a BGP speaking daemon. It's all config.
I don't have a heavy networking background at all. I'm a software engineer that's currently working as a system architect, but even I can understand something as simple as a route distribution system.