Very simple read-only SELECT not run in parallel in postrgresql
The query leader is the only process that communicates with the front end.  All the SELECTed data for the non-aggregated version of the query has to be marshalled up from the parallel workers to the query leader.  That is expensive, so using parallel query here would almost certainly be counter-productive.  You might be able to get one anyway by setting parallel_tuple_cost and parallel_setup_cost to zero.

When doing the count(*) aggregate, each parallel worker only needs to send up one piece of information, a partial count, to the leader.  This is more obviously worthwhile.