- They don't get backed up or synced between devices.
- It's very easy to delete them (e.g, you clear browsing data) or lose them (e.g., you get a new computer).
- Users barely know about them, so they don't know they should "preserve them". For users PWAs are basically just a way to get a launcher for the app, and they have no visibility over locally-stored data.
I think browsers right now are not the right platform to give a future-proof guarantee. Disclaimer: I say this while working on a product that wants to become that platform¹, so I've had the chance of thinking about this problem quite a lot in the past few months.
¹ I wrote a bit about it at https://pscanf.com/s/340/, if you're interested.