Part of the reason for this is that some of Skeema's users are quite large, and larger MySQL users already have in-house solutions for row data migrations. When your tables are huge and/or sharded, a data migration consists of a lot more complexity than just putting an UPDATE statement into a .sql file :)
I do agree it would be good to have some options in Skeema for DML at various scales, and it's something I plan to start approaching in the future, hopefully later this year. Overall my approach with Skeema's roadmap has been to first get DDL right for tables, then get DDL right for other types of objects (finally complete), and only then move on to considering automation for other areas (whether that be DML, or something like managing users/grants, database global variables, etc).
Say I'm adding a gender column to my employees table. I see how Skeema would be an alternative to running DDL in a migration. But I would end up still needing a migration to backfill the data.
So I could use Skeema, but I still need migrations (we also occasionally fix data due to bugs, etc). At that point, we were less motivated to add an additional process.
Is that what Skeema users do, or is there some other approach, or maybe this isn't what I should be doing in the first place?