Most of the time, the offers sent in the mail or by email to you directly are not the best offers out there. Keeping an eye on FlyerTalk, you'll regularly see postings about the newest offers available. Some of them are specifically targeted, with no workaround, but most are available generally. Don't ignore what comes in the mail; just use FT as another resource to compare the offers you receive directly with the offers discussed here.

The credit card companies are making all these offers because for the past three years, the consumer credit market has mostly been paying down debt incurred over the boom years rather than building up new debt and opening new cards. The credit card companies have found mile and point bonus offers a powerful incentive for a lot of folks. And most folks on FT, being savvy and in relatively good credit shape, can take advantage of the largesse without sinking themselves into huge debt. And, because to this point, it doesn't appear credit card companies are completely willing to deny credit applications made by churners (there's been some retraction but not a complete course of denial), it's very possible to accumulate a number of cards with great signup bonuses.

An internet resource, if you want to know more: http://www.frugaltravelguy.com/. There are others, but FTG has been at it for awhile and has funded a lot of travel on credit card offers.