Migration is a complicated beast.
The simple, technical answer to your question is this:
The default behavior network-wide is that all sites start with no migration targets other than their child meta and must request those to be added as a need is shown. Diamond moderators have the ability to migrate to any site they deem appropriate and users can flag a post for migration using the moderator intervention flag if they feel strongly that migration is the best solution for that post.
So, the reason y'all can only migrate to meta is that there haven't been any requests to add other sites. Your mods also don't seem to migrate many questions away, in general, so that indicates that there may not be a general need for migration targets.
The more complicated answer regarding why you might not want to have this is that, in many cases, migration is not a great solution. It's not always the best experience for the user whose question is migrated and there can be some amount of frustration caused on the target site when new migrations paths are opened up. By limiting migrations to moderators only, we reduce the likelihood that poor-quality questions are sent from one site to another.
For example, if a question is asked here that belongs on a Chinese language site, my first questions is whether the question is actually a good-quality question and worth migrating at all. Because the first rule of migration is "don't migrate crap" - if a question would be considered a low-quality question on your site, even if it were on topic, it should not be migrated!
It's often a much better experience for a user to be pointed the direction to the other site and that site's help center so that they can start fresh with a new, good-quality question and, should that question get closed, it will stay put on the new site - which isn't the case with migrations. If a migrated question gets closed (other than as a duplicate), it bounces back to the originating site.
Great migration path experiences often happen when the users on one site are well-informed about the subject matter of the target site as well as their expectations for topicality - for example, if you had someone asking what the name of a Manga was here, that might make sense on first thought to migrate to Anime & Manga - unless you were aware that they have a clear policy that identification questions are not allowed.
I'd also say that, unless you're getting several questions per week that need to be migrated to a site, there's likely no reason to have a migration path. And, well, if you're getting several questions per week about Chinese language, I'd want to better understand why. For one-off events that occur rarely, it's generally better to raise a flag and let a moderator judge whether the question should be migrated.
More info on MSE - Migrate to *any* site? and More options when flagging for migration
