Why do fantasy authors often change their magic systems as their series progress?

How will climate change affect food prices and availability in the next decade?
How realistic are planet-killing weapons in science fiction compared to actual physics and science?
What makes someone turn to astrology instead of traditional counseling during a divorce?
How do tough board exams affect students' career choices and college admissions?
What makes someone choose cultural comfort over financial benefits when making career decisions?
What powers do presidents typically gain during wartime that they don't have in peacetime?
Why do some forms of support feel helpful during grief while others feel overwhelming?
What is the reason again that Trump is imposing tariffs upon our trading partners all of a sudden?