That's because the cables in these application are not single wires but complex micro-coaxial assemblies. (The industry best is 3M Twin Axial.) And that will have much more shielding than an inherently unshielded PCB trace. With an appropriate price, of course.
Note that on balance I'll take something like an OSFP assembly (https://www.te.com/usa-en/products/connectors/pluggable-conn...) with a fancy jacket that pulls all of the individual twinax cables together, but it's also substantially more expensive than a simple flat tape-wrapped ribbon.
Other places where you'll see coax cables ganged together inside a jacket: high-speed USB cables. For example, here's a type C cable cross-section: https://twitter.com/tubetimeus/status/1125926941469462528