From an economic standpoint, AnonymousNecromancer is mostly right for most things.

There's relatively little automation that's been brought to bear on repair jobs, so that repair often involves a lot of time from highly trained and/or experienced specialists. Manufacture, on the other hand, requires much less labor due to automation, and also the labor can be less skilled. There is a whole body of resources and human resources involved with designing the products and the factory in the first place, but you have to pay for everything aside from building the factory anyway no matter how many widgets you produce, and factories are usually relatively inexpensive due to high numbers of products produced.

Mining raw resources is also relatively inexpensive for many resources.

Thus, producing new products often is genuinely less expensive than repairing old ones, especially if the materials involved are inexpensive. This isn't universally true, of course; cars are typically repaired rather than thrown away for all but the worst damage, for example, though we're getting to the point that people are throwing out refrigerators instead of fixing them in much of the West.

What this doesn't catch though is the "hidden costs" of mining and manufacturing, in pollution and other damage to the environment and health. These things are called "externalities" (which is economist-speak for "things that economic theory doesn't handle well"). Things like carbon tax are supposed to help this by putting a concrete cost on things like pollution, so that you can directly account for such things. Say, for example, that your refrigerator breaks. It costs $500 to have it repaired, because you need an experienced refrigerator repairman to look at it, to partially disassemble it, and to bring his experience and training to bear on it. Let's say that a new fridge costs $700. It kinda makes sense to just buy a new one; after all, it's only $200 more and you get a whole new fridge! Which is probably more efficient, quieter, etc. But let's say that there's $300 of "environmental tax" on the fridge. The cost of repair probably only goes up slightly, because there's low or no tax on the repairman's time, so the only thing affected is any replacement parts. Now a new item is twice as much as the repair instead of 30% more, so you'll probably keep fixing things.

Of course, there's more to the story - for example, would it be better from a resource consumption perspective to just kill the repairman and make new fridges? But on the assumption that we allow people to live and more or less to live as they wish, this isn't a very relevant train of thought.