When publishing a paper/book with a certain publisher there is an interaction between the prestige of the publisher and the prestige of the author: An author publishing with prestigious publishers is considered prestigous; a publisher that publishes prestigous authors is considered prestigous.

Since publishers are commercial entities, their prestige is an asset they want to protect.

So there are other incentives at play when it comes why citing ArXiv is currently not en vogue.

I always find it refreshing to see how pre-print driven reserach communities like physics operate in comparison.