Many don't because that type of work doesn't directly and obviously drive revenue, doesn't have slick customer-facing UX, and isn't on the radar of product or management. For every shop that actually has their act together there are two that fall into this trap; from what I gather consulting and contracting places in particular are quite prone to this line of thinking since the distinction between billable and non-billable work underscores this further and leads to especially perverse incentive structures.