Because creating debs is largely a completely distinct undertaking from the dependency and build management the developer of an app does.

Bundles, whether via images or static binaries, allow app developers to distribute their app against the exact dependencies it was developed against -- potentially using the same build system.

There's obviously tradeoffs to each approach, which is why I don't think there's one right way to distribute every bit of executable code on a system.

> People want containers to be magic but they're actually a hard problem.

I work on a container orchestrator, so I understand some of the difficulty. :) Mobile apps are years ahead of desktop apps when it comes to containerizing in a user friendly way. Obviously there's plenty of work still to be done, but the problem is far from intractable and the benefits are enormous.