hi lebonche,
first of all: welcome aboard!
to your question: i agree that there are usecases for this feature and this is also discussed in the team from time to time. however, not so long ago, we decided against it.
main reasons are the maintenance efforts across the supported operating system versions for comparable few benefits. see tune down usage of screen-lock by r10s · Pull Request #1627 · deltachat/deltachat-android · GitHub for more and detailed reasoning (that time we had a lock screen - so the effort i mentioned is not only theoretical 
)
that said, things are always moving, and there were some efforts in an at-rest encryption, so, might be that by that something will come back (maybe with fewer UI involved ). but there are no concrete plans for the foreseeable future.