The range hood aperture should overlap the rising and expanding cooking plume for good capture. The air flow rate should be sufficient to ensure containment of the plume captured in the aperture such that it doesn't "reflect" out into the room, but is instead ejected into the outside environment.

The range hood should also be affordable and aesthetically pleasing, fit into the available space, provide a fire block, be reasonably quiet, and support cooktop illumination.

Most of the time for most of the people looking for a hood, not all of these "shoulds" are simultaneously able to be met. I can only advise on performance "should's"; the affordability and aesthetics "shoulds" have to be weighted by the buyer within his own trade-space versus grease and odor removal.

Ideally, then, for the hood aperture to overlap the pans on the front burners by at least ten degrees at the height chosen for the base of the hood, the depth has to be roughly 27 inches front-to-back, although 24 inches may be satisfactory for most, and in a wide range of candidate hood styles, the largest depth available. Arguably, 24 inches as a wall mount is likely as effective as 27 inches in a peninsula/island mount.

It is important to keep in mind that good containment air flow is
nominally 90 CFM per square foot of aperture, so larger hoods need more
air flow, and what goes out has to be replaced, so higher make-up air
capacity is also required. So size is a cost driver.

As to why many hoods do not meet the ideals of sufficient depth for good performance -- this is determined by market forces. Even if everyone who wanted a hood came to this forum and studied all the commentary, treating it as canonical truth, many would have to choose a lower performance hood design for cost reasons, or due to other factors.

If the greasy cooking is kept to the back burners, then a 20-inch hood may be sufficient.

kas