From a personal perspective, besides the many aspects already mentioned by others, there's also the simple lack of challenge. I've already mastered an x-number

From a personal perspective, besides the many aspects already mentioned by others, there's also the simple lack of challenge. I've already mastered an x-number of technologies in my 25 years as a developer, I'm 100% certain I can deal with the umpteenth new one (albeit a bit slower than I used to). I already know I can program, and what my strengths and weaknesses are in that area. I'm more motivated to take on challenges I might actually fail at, or learn something new about myself.