Woo hoo! I want to join in. Like somebody else, I'd like to have a campaign world that is "realistic" or at least "rational". Now lots of thigns are in the rules that might confilict with this goal the way you see things. The choices are to either change it to something you think is better or come up with a rationale for them. If you change them, great. PCs don't need to be reading the MM and arguing with you on what goes on in your world anyway. I think the intial question is valid, why should various elemental based creatures always use like attacks? Here's some of my rationales:
1. In the case of extra planar creatures, they probalby have like elemental weapons and attacks because everything from their plane is made of the stuff. Sure a fire elemental might have a sword of fire, his butter knife is probably a +1 flame tounge also. Also, keeping things that aren't of like elemental types in such a harsh environment might be hard too.
2. It may be due to the manufacture of such items. Although the method for manufacturing such items described in the rules calls for a certain amount of gold and XP, it's doubtful if the maker of the magic time absorbs all teh gold peices into the magic item. Instead they're used to buy materials and services needed to work the magics to make such an item whch are reduced to a price in the rules to simplify things. It may be that the makers of such items have more access to the raw materials needed to create such cold based items than the materials needed for normal or fire weapons. So, while the "objective" price of such an item may be higher, it may actually be cheaper to make alike item because getting the resources to make other types would cost more due to regional supply and demand.
3. Their main adversaries aren't like typed creatures. Cold giants may be surrounded by cold creatures, but those creatures really aren't their main cause for concern. All the creatures that are dangerous to them in the region have been "cleared' long ago and what needs to be dealt with are "mosnters' from other areas. When they go raiding, do they raid the snow bunnies warrens*, or do they head down into the human lands and raid the local villages and caravans? Simply put, they have liked based weapons because they actually do spend more time fighting opposed type creatures.
4. Religions and culture. Anybody can justify just about anything by saying "this is how things have always been as long as they have been passed down from father to son since the begining of time". Of course, then somebody breaks tradion, comes up with a new technology or tactic and wins the battle or war. Then everybody else plays catch up. So, either their is no advantage to such a change or they haven't come to that change yet (see 5). A DM could always play a campaign where such a tactical revolution is taking place and an army with superior weapons are forming an empire.
5. The spell caster with the recipe graduated.
*In my world snow bunnies are horrible 8' creatures with huge, sharp fangs with cold attacks that can bite your head off whole. They also enjoy riddles and will grant safe passage for carrots.
