[killyourcar]

The actual reason is that long running invocations run synchronous workflows, typically requiring holding threads and sockets open for the entire duration of execution.

Lambda is a complex system, and holding those sockets for long times across many services could cause resource starvation issues. You've got load balancers, data plane, control plane, tenant vms, and a whole bunch of caches, and support services that all need to be ready to roll over the lifetime of the invocation.

And you have to consider the use case for draining and patching lambda pools. If someone is running a two hour function and you need to take down any server that's currently holding a thread or socket for it, you need to wait for the function to complete. You can't start a new load, so you are really inefficiently using resources until the function completes.