In Spelunky, aside from finding the rare crates the only way to get items is by giving blood sacrifices which eventually also gives you an item which raises heath for collecting blood, or by buying them from the shop owners. The shop owners have a lot of items, they can pack a lot of damage and the sacrificial altars always seem to have fresh blood on them. Now put two and two together.
It could even explain why they're so quick you kill you for even the most accidental of slights, malicious in intent or not. They've seen adventurers come and go hundreds of times; who knows how many times one of those opportunistic little bastards (You?) have tried to steal something (or worse) from them? It's only natural that, in time, their trigger fingers would get a little jumpy...
Whilst it is vaguely logical that the value of loot rises as you go deeper into the caves, answer me this— why is a single gold bar worth $500, but a stack of THREE gold bars only worth $1000?
This is addressed in the Xbox Live Arcade remake of the game, where a stack of three gold bars is now worth 1500. If you whip the stack, you'll see why - it's actually three individual gold bars arranged in a pile. Clever!