I think it's basically because people like to collect things, and they like to have what other people don't. Thing is, eventually everybody will have it. I expect Pokémon to lose interest shortly after Christmas time, unless they come up with something new to continue interest. There will soon be a lot of knock-offs on the market enough to make it a dying fad, so keep that in mind when you're buying/investing in them. That being said, here are a few tips for all those people trying to "catch 'em all..."
Some people may question why I put so much effort into this, considering I won't make a lot of money, or that it's a kid's game, or something -- but to be honest, I don't have the capital it takes to play the stock market, so I deal with internet auctions and collecting comic books and trading cards. I'd like to see someone else have a fun hobby that also makes money!
A lot of people are already snatching up all the Pokémon stuff they can find. By Christmas, expect to have shortages of a lot of things kids will want, just like the Teletubbies, the Tickle-Me-Elmos, and (if you are old enough to remember) the Cabbage Patch Kids. So if you see it (and especially if you don't see a lot of it), snatch it up now.
One of the hottest things now is the Pokémon Trading Card game. Thing is, you can't find the cards unless you pay more than retail price (Retail is $3.29), or unless you scour the country for them (and we all know your time is worth money). So here's my secret: go to ToysRUs.com and buy them there! Right now, there are no shipping charges, and you pay retail price! So you've instantly save yourself the hassle of running around town and paying more money for pieces of cardboard! Plus, with each order, they're giving $10 coupons, so you save even more!
Another good place is the Internet. On Ebay, people sell cards, dolls (okay, "action figures" for all the macho guys out there), and games, and if you time it just right, you can pay less than retail for things. But that involves you knowing what you're buying, as well as trusting whom you're buying from (just follow the standard internet auction protocol and you'll be okay).
And if you know where to find them, go to the Flea Markets. If you want to buy a lot of cards, buy the sealed boxes. Sealed boxes are actually worth more than the individual packs combined. I bought a sealed box of 36 packs for $100, and it would sell for ~$140 dollars right now on the internet. Just keep in mind, that potentially you could have one of the cards that are individually worth $35+ inside one of those packs, and you might sell it for less than it's actually worth -- that's the added "selling value" of the sealed box.
New Note, 10/25/99: I've been buying a lot of booster packs at retail price. In the long run, it's cheaper to find a wholesaler/reseller and get your cards from them, but if you're looking for a couple of packs here and there, and don't want to have to fight the crowds and hoarders out there, try this neat little trick. Most businesses have a price matching policy, especially one that does not have a disclaimer based on availability. They usually sell the cards for more money, hoping to make a profit. Today, I called them on their policy and was able to purchase 25 packs at the retail $3.29 price! I don't think too many people know about this one, so take advantage of it before the retailer gets wise (or worse yet, before other people buy up all the cards!) This is a great way to purchase the Fossil sets as well!
There are a lot of sites that maintain pricing guides on what the going market is for the merchandise out there -- this site is not one of them. And keep in mind that there is a Japanese version (they're called "Pocket Monsters" and most of the text is in Japanese) and a U.S. version (this is what the television cartoon series is based upon and the test is in English).
If you're buying stuff hoping to collect or sell them later, be sure you decide now when you plan to sell them back to the public. Once something goes past fad status, you will hard pressed to find a strong market for what you've hoarded. I plan to release most of my stuff near November 12, which is when the Pokemon movie comes out; that's when I think the market will be the strongest.
So buy only what you feel you can afford, and what you are comfortable with storing away for an indefinite amount of time if no one else wants it. Don't buy several packs of cards unless you are okay with building a collection for yourself. I still have my entire collection of "Death of Superman" comic books that are allegedly valued at $300+, but I can't find anyone willing to pay me that kind of money for them. Maybe in a decade or two...