Ydars said:
Derren, part of the tax they exact from the settlements they visit is their food and fodder for the animals; they don't carry much food as it is wasted space.
Which is quite dangerous. if the village they come to is destroyed or does not have any spare food left the caravan has some huge problems.
Caravans might present a nice target but the power curve for monsters is pretty flat in 4E so even quite a powerful monster is going to have problems taking on 50 guards because I am envisaging a caravan of 30-50 covered carts pulled by horses. Of course a band of monsters will manage to overpower a caravan every now and then but this is part of the setting and what makes it so dangerous (and fun).
Considering how much it costs to create such a caravan (soldiers, weapons&armor, carts + spare parts, horses, supply, trade goods ) loosing one of those would be a huge blow to the entire area. And 20 monsters should not have too much trouble with 50 guards. After all they don't have to kill them. They only need to kill some draft animals which forces the caravan to abandon some goods.
Ydars said:
Also caravans could travel on rivers in boats and gain some protection that way.
From previous Monster Manuals you should know that water doesn't protect you from many monsters.
You also seem to assume that monsters will not form civilizations of their own and thus compete with humans on a large scale. Especially humanoid monsters are problematic, for example Hobgoblins. They are monsters, which means strong enough to challenge PCs which means they are stronger than NPCs and their phisiology is very similar to those of humans (or other player races) which means they also want those fertile spot to build cities there.
Add the traditional militaristic focus of Hobgoblins and that they are likely not nice so that the PCs can fight them it doesn't look good for human villages.
This situation reminds me a bit of Ruanda or Dafur where the monsters are death squads, the defenseless villages are still being defenseless villages and the PCs are the only merceneries willing to protect those villages in a 100 mile radius.
And because there are different races involved the monsters would not hesitate to perform genocide on humans.