If your concern is about your own health as an individual? I want to lose weight now? I think it's useful to say "I have agency, I can lose weight". If you have the agency to lose weight, it makes sense you can take on the responsibility for having gained weight.
"Why did society in general get fatter?" is a different question. And if your main concern is "how can I lose fat?" it's not directly interesting.
If your concern is about your own health as an individual? I want to lose weight now? I think it's useful to say "I have agency, I can lose weight". If you have the agency to lose weight, it makes sense you can take on the responsibility for having gained weight.
"Why did society in general get fatter?" is a different question. And if your main concern is "how can I lose fat?" it's not directly interesting.