I'm sure there are lots of reasons, but past 18-21 or so you start going downhill. I am not the kind of guy who would be a muscular 250 pounds without doing a damn thing, if I was then perhaps I'd have the genetics to be able to lie down flat and hammer curl 150s without thinking "damn this is a STUPID idea."
It puts a trememdous amount of stress on the pecs, shoulders, and biceps. I probably have some of my chest stretch marks from doing just that. If I dont sit up on the bench, I try to skillfully throw/drop them when nobody is around. I still think its risky, so I much prefer dumbells on the incline and barbell flat/decline work. Or if a couple hefty spotters would be perfect, but most of the guys in my gym weigh at best what those dumbells do each.
There are people who deadlift 600+ for reps who feel they are "fragile" and therefore dont try for more. Listen to what your body can handle and go with it, if you are flat benching 60 pound dumbells I dont think the risk is anywhere near when you are 2+ times that amount. There are days when I feel I could throw 315 while lying down on the flat bench, and I'd be willing to bet I could. I feel the dumbell is a lot more dangerous because you lack all that support by combining limbs. Plus obviously, I am not straining myself in the same manner. Lowering a dumbell effectively behind me, while using very heavy (for me) weights, while theoretically close to complete failure as it is, doesn't seem in the least sensible. If you are one of those guys who trains to 80% failure and thinks you are done for, and using little weights, then sure its probably feels OK.
When I worked in warehousing back in 96/97, I used to carry TV sets over 100 pounds on one shoulder, walking up a rickety ladder roughly 15 feet to the top, and then threw it on a rack. I would jump from the racks all the time, nowdays with my knees you wouldn't catch me jumping from the first step of the ladder on MOST days just because I dont push myself more than I have to. We take less risks as we age, because of warning signs that come up from time to time. So while I'm far stronger than I was 10 years ago, I'm a lot more sensible to what I do with myself, because I've been warned many times, and the risk is a lot higher with the weights PLUS combined with my increased age.
Two big things working against me. I'm sure lots of people can think of things they did when they were 15-25 that they wouldn't even laugh at now because it would be so scary. And like right now, I need more sleep. Most of the days where I feel like chewing on a tire iron are gone.