Because as you age, your once challenging-but-can-work-around-it workout will become tougher. You'll have responsibilities at your job (I hope for you), a wife, kids, etc. You'll sleep less each night, without knowing why (I'm lucky if I get 6 hours these days). Nothing original really, just what we call a life. I just don't believe that you can lift heavy if you don't dedicate a good chunk of your life to it, be it diet, rest/impact on social life or gym time. Whereas if you train volume for beach body, or are just fucking around at the gym with no attention to your diet etc., there is less chance you'll get some injury. You won't get much results, sure, but if it's just a hobby why shoot for the stars if it put your health at risk?
I just don't know anyone who hasn't one nagging injury past 40 yo. I'm not just talking about injuries in the gym. I'm talking this motorcycle accident you had when you were 15? Well up to 40 you were good with it, now all of a sudden you'll start feeling it again on rainy days and shit like that. No fun at all. Adding to it by putting heavy load on my back? Yeah right!
I've also been on this site 10+ years so I've seen my share of "just squat!" bros that will say squat is great for you and if you feel pain, you're doing it wrong. Well guess what, not all body types are right for squats! Yeah I could spend 2 hours a day with a trainer to maybe try and fix some mistakes, but I don't think I'm getting away with bad knees due to being obese in the past and levers/body shape definitely not designed for squats.