I rescued this comment from another thread. It was made twice by two different people so it's worth addressing in its own thread. Please feel free to chime in.
First of all, how farmworkers, dockhands, etc. got strong has little to nothing to do with the fitness needs of a 20-70 year-old city dweller. As a trainer, it should be obvious that when someone comes to me for training, I can't suggest that they quit their careers and get a job unloading ships. Not gonna happen.
Second, many of those workers are strong, but not healthy. They suffer usually repetitive stress injuries from doing the same motions over and over again that break their bodies. Not the same as modern fitness protocols would have you train.
Third, water bottles and big bags of rice aren't as compact as iron weights. It's an issue of volume. You could fit 2-4 of my kettlebells in a one square foot space and get a lifetime of use out of them. A 40kg sack of rice is going to be significantly more bulky. And I seriously doubt you're going to find water bottles that big.
Fourth, these irregular objects are great for training in some ways but they don't offer the easy measurability of weights. When you lift weights, you know exactly what you're lifting and a progression is easily planned. With irregular weights, it's difficult to add or subtract weight, making them less than ideal primary strength tools.
It was also mentioned in that thread that body weight training could be had at a cost less than that of buying weights or joining a gym. Of course body weight training is great. I do it all the time. But not everyone is strong enough to work with their body weight in the variety of exercises that they might need. Weights allow a progression to the work that is hard to beat. They can use a weight significantly lower than body weight to get strong initially and use body weight if they desire when they are stronger.
The kettlebell protocols that I use mix weights and body weight training so that you can go beyond just using your body weight -- an issue that stronger people will encounter fairly quickly.
I can think of 3-4 more reasons just off the top of my head. But this will suffice for now. Needless to say, water bottles and bags of rice aren't about to replace weight training in any form any time soon.