Why do many players resign from chess in a relative young age? And become coaches or writers instead? Have they reached their peak rating and find it unmotivating to pursue any further?
Are they lacking financial security?
Are they tired of travelling?
A lot of chess players make a living off of chess coaching. You can't make enough money by playing tournaments unless you're like top 100 in the world. Therefore I assume that some people must have to resort to writing books and coaching people in order to make enough money. Considering that they resign at a young age, they must plan this out. On top of that, I would assume that maybe when you combine family life (if you're a father or mother) with coaching, having a small normal job, and a tireness from travelling, then you'll never have time to play at tournaments.

By the way, I would say that no, they haven't reached their peak, that's not why. Just look at Mark Dvoretsky, he is only an IM, but he could definitely be a GM if he wanted. It is because of personal choices, financial reasons, and convenience reasons. Or, maybe they just love teaching but hate playing in serious competitions due to stress. Besides, tournaments can cost tons of money.

Chess is considered the second most stressful sport behind skydiving.