Vast majority of consultants are working on engagements where "the deal" was negotiated weeks/months earlier.

The goal isn't to eliminate flexibility, it's understanding probabilities. If fully-flexible/refundable flights are 3x the fare of restricted, then in aggregate, the company could have a chunk of consultants throw out their original reservations and rebook later flights, and still save money. Yes- sometimes consultants need to stay longer than planned, but in an age of prioritizing "work-life balance," most consultants are encouraged to stick to their schedule and get back home when originally planned.