Parallel Aggregation support for aggregate functions that use transitions not implemented for array_agg

It seems CREATE AGGREGATE was expanded in 9.6 to support parallelization of
aggregate functions using transitions, with the addition of serialfunc and
deserialfunc to the aggregate definitions.

https://www.postgresql.org/docs/10/static/sql-createaggregate.html

I was looking at the PostgreSQL 10 source code for some example usages of
this and was hoping that array_agg and string_agg would support the feature.
At a cursory glance, it seems they do not use this.
Examples I see that do support it are the average and standard deviation
functions.

Is there a reason for this or it just wasn't gotten to?

Thanks,
Regina