Why would many people have degenerative disc disease and others do not?

Same reason some people have diabetes and some don’t. Or rheumatoid arthritis. Or cancer. A complex combination of genetics, environment, behavior, and luck.

Exercise might also be one of the factors. Perhaps they’ve never even tried to touch their toes. Flexibility and exercise while younger can forestall the onset of some degenerative disease later in life.

Or was this just a question where you get to show off by mentioning that you can touch your toes (presumably while standing and bending at the waist and not bending your knees)?

Yes, @CWOTUS . As a grandmother of three I’m just showing off my superlative health status.

You probably meant it jokingly, but degenerative disc disease most definitely isn’t because they need a new mattress. It probably also isn’t because of stress at work for them, either.

I’m with @syz – it’s some combination of how you’ve lived your life, your genetics and plain ol’ luck of the draw. I know a guy who had a nearly fatal car accident in his mid-twenties. He’s struggled with cognitive issues (looks like senility) from the brain injury (and facial reconstruction came with nerve damage, too) since then..so his 60 looks more like someone else at 80 or 90.

To make you feel really good, I’m only 41 and I cannot reach and touch my toes – so you’re in better shape than 41!
...but I’m okay with that because I know I can’t touch my toes because of scoliosis and my unique physique (I’m all legs. I’m incredibly long-legged, high/short waist-ed).
I danced/trained in ballet in University and worked incredibly hard to have really mediocre range of movement—so you probably have better flexibility than I had even in my 20s, doing 25+ hours of dance training each week.

Everyone has different genes and have been exposed to different things. Different nutrition, different environmental hazards, and have suffered different injuries. Minor injuries that might not seem significant can still cause damage.

Even when I was a little girl I was never flexible. I took ballet like @geeky_mama and did gymnastics and for a very brief time when I was around six I could do a split if I was very careful while other little girls could just fall into one who never even did anything specific to stretch their muscles. I never could touch the floor if I stand up straight and then bend over, or touch my toes when I sat on the floor with my legs straight in front of me. Some teachers start with stretching on the floor, which is impossible for me and can cause me an injury. I know have muscle troubles at age 45, it has been going on for over ten years, which seem to be at least somewhat related to lack of vitamin D. vitamin D has been like a magic oill for me, I really think I was on my way to being a cripple or developing MS or Parkinsons or some other neuromuscular problem if it had not been discovered I was so deficient. Maybe degenerative disc troubles also have a correlation with vitamin dificiencies? Who knows. I think my thyorid and low iron affect my health in many ways, including muscles and tendons and other parts of my body.

Simply, genetics and environment.