At the risk of sounding dreadful, have a question about 'wp'

For seasoned product managers that have used wp 'wordpress': many developers over there always throw around some advice about avoiding the need to write join queries because its very overhead intensive.

To contrast with non-wp products, joins are use extensively and importantly. Why would they give out this advice to avoid them in wp? Could it be because from within wp, SQL table design and indexes aren't crafted carefully?