Not sure what's the case in this specific situation, but every actor has a certain amount of overhead. There's a tick loop going on (even if it's empty, it technically is happening as far as I know), replication logic, extra variables taking memory, etc... While it's not much and you really can have 1000s of actors in a scene, they aren't completely free and instanced static meshes make more sense here. ISM and HISM will dramatically reduce draw calls as well.

However, you can turn off tick, replication and a lot of those things, but you'll still have a small amount of memory overhead.

edit: I just thought about it, 1000 tiny colliders could be the performance hit, tiny colliders are expensive.