The number of migration targets available for users is restricted, mostly to avoid incorrect migrations, as such migrations are difficult to undo and also impact another community.
Moderators can migrate to any site in the network. If you want to migrate somewhere else flag for moderator attention (flag, other) and suggest it.
The exact targets available for migration are chosen by SE, informed by precedents and discussions with the respective communities. If you notice frequent need for migrations to a particular other site, feel free to propose it. Usually, sites that are still in beta are excluded. Note though that the total number usually does not exceed four, thus the list will stay short.
The path to MO is indeed asymmetric, which is unusual. I am not certain why the path to MO is asymmetric. Michael Greinecker provided some context that recalls concern over migrations to MO in this community, another factor might be concern on MO over incorrect migrations.
But sometimes questions also get migrated from here to MO (by a mod). Indeed it is the most frequent target site (with twelve migrations over three months).
You can find some data about migrations on the respective tools page (link available to 10k+ users).
As one can see from that page overall migrations are pretty rare, e.g., there were only six over the last three months to the statistics site, and around 35 total. Thus, to require moderator involvement is not a problem.