Why don’t large aircraft (larger than ~737-size) have even partial manual-reversion capability? Essentially all large aircraft1 have flight control surfaces powered, either directly or indirectly, by hydraulic actuators, as do most medium-sized and some small aircraft.  Small- to medium-sized aircraft with hydraulically-actuated flight controls essentially always have some form of manual reversion capability, where the primary flight control surfaces (always the elevator and ailerons, and sometimes the rudder as well) are operated manually by the pilots, usually by control cables attached to the pilots’ yokes and pulling (depending on the aircraft) either on the control surfaces directly or on servo tabs which then generate aerodynamic forces which move the control surfaces.  This allows the flight control surfaces to remain operational in the event of a total failure of the aircraft’s hydraulic systems, albeit requiring the pilots to exert a (sometimes considerably) greater force on the controls to operate them.

Large aircraft, on the other hand, generally have no manual-reversion capability at all.  This is ostensibly due to the greater aerodynamic forces acting on their flight control surfaces at cruising speeds, which would require superhuman strength to completely overcome (despite the fact that, before hydraulic flight control boosting came into wide use, many, many large and fast aircraft were successfully flown with purely manual controls through the use of a number of force-reducing tricks).  However, in a no-hydraulics situation, having any manual control capability at all, even with a significantly- to considerably-reduced control authority and range of movement compared with hydraulically-boosted operation, would be an extremely-useful adjunct to throttle pitch control and steering (which – especially throttle steering – are difficult, sluggish, clunky, and only really usable for coarse altitude and flightpath adjustments).  So why don’t large aircraft have at least partial manual-reversion capability, despite even a little flight control authority being far, far better than nothing at all?



1 I'm using "large" here to mean "larger than a 737 MAX 10".
Adding in manual control mechanics for a large aircraft has serious drawbacks:


Weight: For a large aircraft, the weight of pulleys, cables, etc., will add up quickly. This costs money in fuel and by reducing the useful load of the aircraft.
Complexity: The manual system would have to be routed through the entire aircraft, including passing through pressure bulkheads and other structure. This takes a lot of time to design, install, and maintain. While there are certainly ways to deal with the high forces involved, that just adds more complexity.
Vulnerability: There have been multiple accidents in which manual control systems were improperly set up or jammed. The manual system would have to be designed so that fly-by-wire system would still work even if the manual system develops an issue. If an event is catastrophic enough to cripple all of the hydraulic/electrical systems, there's a good chance a manual system would be disabled as well.


The alternative is to design a hydraulic system that can match or exceed the reliability of a manual system. Of course this is not without its own complexities and issues, but virtually all modern airliners have gone this route. The weight and complexity of a manual system are not worth the remote probability that such a system would be both necessary and useful.