The state is enormous. Dedicating megabytes and megabytes to key state is painful. And so is tracking state across components and through distribution channels. If you’re not afraid of that then just use symmetric crypto and be done with it.
To be clear my comment is specifically only relating to signature schemes, not encryption.
> The state is enormous
The scheme I linked to points towards efficient "pebbling" and "hash chain traversal" algorithms which minimize the local state required in quite a fascinating way (e.g. see https://www.win.tue.nl/~berry/pebbling/).
> tracking state across components and through distribution channels
Assuming you have reliable ordering in those channels I don't see how the stateful nature of such schemes makes it hugely more complex than the essential hard problem of key distribution.
