I thinking dumbing down is pejorative and used incorrectly in the industry. Not everyone has the same capability for hand eye coordination and reflexes and that doesn't make them any worse. The fact that there are different markets to serve should be applauded. Our jobs as game makers is to entertain people - you choose who you entertain.

Difficulty levels. I could never understood why they so rarely make diffuclty levels (and by this I mean more than the standard 3 easy/normal/hard, because it's a bit not enough). There are some games (genres) I beat at hard and some I can't beat on easy... Especially, since such difficulty levels are frequently quite easy to implement (reduce damage from enemies for example).

Depending on the game mechanics and what options they have to make things easier/harder they dont want to have to test/debug/tune too many difficulty levels (or possibly worse a continuous slider difficult controls effects)

Some games it might only ge adjusting enemy hitpoints, hit probabilities, target AABB or damage-to-opponents/damage-to-players multipliers. But not all games have factors that linear and modal changes can cause pendulum swings (or open up loopholes) which need to have many other things adjusted to keep the game playable (or to achieve the desired REAL difficulty)