Similar reasons people buy overnight sleeper tickets from the west coast to asia for sky-high prices. That person has to be there, and they have to be rested. If they are negotiating 100m+ dollar contracts its a no-brainer.

Intercontinental business-class tickets have a tangible benefit, guaranteed lie-down seat and relaxing accommodations that enable rest, as you correctly point out. Flexible airline tickets have no tangible benefit, it is simply risk mitigation (avoiding wasting a nonrefundable ticket if a person can't make a flight) and like other risk-mitigation products such as insurance, extended warranties, car rental protection, etc, it is priced so that the cost exceeds the benefit.

And as stated elsewhere, the majority of consultants are relatively junior people whose role has nothing to do with negotiating contracts.

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.