The accountants gave the designers at Quanta, Wistron, etc. a price target and a set of specifications. The ODM met the design requirements and the price targets by cutting corners where they could, and the design was accepted and put into production. That's how it works.
The vast majority of Inspiron, XPS etc. buyers don't need gigabit - so they'll never notice. Just as leaving the sheet metal behind the dashboard primed but not painted on a Hyundai Accent - it lowers costs and just about no one will ever notice.