Difficulty selection looks like a simple choice between easier and harder, and it is one of the more awkward problems in game design. A single option has to alter systems that interact in ways players never see.
The slider adjusts several things at once
Raising difficulty typically increases enemy damage, reduces the player's health, changes how quickly opponents react and alters how generously resources appear in the world.
Those changes do not scale together. Doubling incoming damage may make a game marginally harder in one section and effectively impossible in another where the player has few options.
The result is a setting that feels inconsistent, because a single control is being applied across encounters that were built with different assumptions about the player's position.
Simple scaling produces the wrong kind of difficulty
The cheapest approach is to multiply enemy health and damage, which makes fights longer rather than more interesting and rewards patience over skill.
Better approaches change behaviour instead, giving opponents more aggressive tactics or removing telegraphs, but that requires designing multiple versions of the same encounter.
Testing cannot cover the combinations
A game is usually balanced most carefully around its default setting, because that is the experience the designers intend and the one most players will choose.
Other settings receive proportionally less attention, and the highest difficulty in particular is often tested by people far more skilled than the audience selecting it.
Combinations multiply the problem further, since a player may reach a difficult encounter with any of a large number of equipment and ability configurations.
Difficulty is not one quality
Players find different things hard, including reaction speed, resource management, navigation and the tolerance for repeating a section after failure.
A single scale collapses all of that into one axis, which is why granular options separating combat, puzzle and navigation challenge have become more common.
Some games treat difficulty as the subject
Games built around a specific level of resistance often omit settings entirely, on the grounds that the intended experience depends on everyone facing the same obstacle.
The argument against that position is one of access, since a fixed difficulty excludes players whose limitations have nothing to do with willingness to persist.
Recent designs have tried to satisfy both, offering assistance that is separated from the challenge itself so the encounter remains intact while the barriers around it lower.