Because almost everything you've seen as 'working code' started its life as an announcement.
And because to coordinate and discuss future work, there would need to be some announcements.
And because some announcements (based on the persons, e.g. here GvR is involved) or the funding (e.g. here MS is involved) or the specificity (e.g. here 3.10 timeframe is discussed) are more important than others.
>Meanwhile the Erlang people quietly produced a JIT without any advertisements.
Good for them. That's maybe because much fewer care for Erlang (and thus for the advertisements) related to Python (which has a much larger dev base), so the advertisements of the former are posted fewer times and discussed by fewer people.
