Why is meta_key nullable?

In the wp_commentmeta, wp_postmeta and wp_usermeta tables, the meta_key column is set to be nullable by default.
As a key-value store, surely null cannot be an acceptable key?
It may only be a small overhead for small sites, but the query performance on large sites(>5gb db) from having this column not be null would definately be benificial.
Is there any use cases where the key would be null? Can you even read/write to a null key via conventional update post meta calls? If not, is there a process for creating a change request?
I get meta_value being null, but not the key.
Thanks