Because that would solve that particular situation, but wouldn't be good for the kid overall. Situations like these are where kids learn introspection (what is it I actually would like to do now?) and emotional autonomy (I can create a good experience by myself), resulting in the idea of self-efficacy, which, beside the cognitive abilities, is the skill to have to successfully navigate the stressfull and anxious situations of adult life.

Think about you adult life: When are the moments you have some kind of epiphany about something that currently bothers you? For many people this happens when out for a walk, under the shower or a similar situation when the own mind is free to wander.

Cognitive development is important, but school, discourse with parents etc. is usually enough for the healthy development of most kids. For a successful life as an adult, the emotional development is just as important, though.

Because it’s not your job as a parent to entertain your children.

Or let the kid plan an activity. Sesame Street literally has a song about this.