They're hardware projections into your living space of a massive system run by Amazon. It's the massive system that open-source will have trouble replicating.

Most people use Echos as voice controlled music players with occasional smart assistant functionality, this shouldn't be too hard to replicate in OSS. You could argue that the extend to which they're not making you buy into the Amazon ecosystem is a major failure of the product line.

Spotify multi-speaker playing + a LLM answering questions would cover what 80% of people need.