Why are leather goods so expensive?
To make leather you have to kill the animal - and animals are more expensive to grow than plants (in general). Compare the price of meat with vegetables. And something made from fibres can be much lighter, so the same weight of fibre is going to make a lot more things.
Are they really that more durable? Generally speaking, yes - although very expensive types of leather (because they tend to be very thin) can be a lot less durable. And it depends on the application. Because leather is one piece of stuff (like paper), it can be cut more easily than woven fabric of similar weight. On the other hand, it doesn't wear as easily (for the same reasons), and it has no holes, so it can be good for holding powders and other things that move through woven containers.
Will leather disintegrate after being regularly exposed to the outdoors (city in the tropics)? Yes - moisture is a problem with leather, so the hot humid days you have in the wet season make it much less durable. It needs to be oiled often in such climates, and even then doesn't last all that long. However, in cooler climates, leather is long lasting.
Are leather goods worth it, from a Mustachian point of view? It depends on where you live, and what it is used for.