[lowpro]

Because although you don't have source code (like other commenters are saying), reversing a program to get into a company would be the hardest way to go. Red teams are used to test a company's overall security, and reversing normally wouldn't make sense compared to phishing, using common exploits, and owning the network. Reversing binaries is not the job of a red team, but pentesters of specific systems.

[PeterisP]

Red teaming isn't limited to "get into a company" testing of networks, it's also used for testing products and infrastructure that's outside the company. For example, you can reasonably have a red team evaluation of some authentication or payment infrastructure based on smartcards or mobile apps, and that'd inevitably include reverse engineering of all the artifacts that are available to the users; and in such cases also likely that many/most software parts of "your" product or device aren't made by you but redistributed from some other vendor, and you don't necessarily have the source available for that.