A totally valid point, but I'd argue those should be handled by a separate tool or process. Data migrations tend to be fully programmatic; tools and frameworks can help reduce the code required, but cannot handle every possible case. (having performed numerous multi-billion-row data migrations, I learned this painfully first-hand...)

For simpler cases, where it may make sense to run a data migration immediately after a schema change, a good generic middle-ground may be configurable hook scripts. A declarative schema management system can then pass relevant info to the hook (which tables were changed, for example) and then the script can run any arbitrary row data diff/apply/migrate type of logic.

I do understand your point though; for relatively straightforward data migrations, an imperative system can capture these much more cleanly by just coupling them with the corresponding schema migration code.