Why not delete by default or after 30 days or something more user friendly...
I assume storage of this data for them is nearly 0 cost, reputation is not an issue as most >99% of people seem to simply don't care about privacy and there is probably no legal limit.
There is no reason for them to do it. In fact if they did, it would be considered a loss of value. Its valuable dataset.
Even deleting stuff in 'big data environment' is generally considered too expensive rather than just un-indexing data points that are not relevant anymore.
There is in the EU. Keeping data indefinitely is illegal. Expert opinion varies on how long you can keep it, many think you're save with 12 months, most think 36 months is too long.
So "Alexa, order more beer" in itself can be kept for as long as you want. The audio recording may be a different matter because of voice recognition.
As to how long personal data may be kept, well in the UK one can sue on a civil matter for up to 6 years so there is a good argument that anything related to the service you provide and in relation to which you might be sued should be kept for 6 years. I expect it's similar in other countries.
One day it will all be compromised or misused, because that's what history teaches us.
They just need to destroy that link and can then keep the transcripts for as long as they want.