The problem is that they did see it clearly in the short term, and more ads and tracking always provided considerably more revenue. If Outbrain offers you $1M to be put on your site but it slows page load by 500ms on average, you put Outbrain on your site because there's no easy way to quantify how bad 500ms is to a userbase of 100M.
It sucks, and is part of the reason I changed industries.