There are many reasons, but I'm going to assume the person in the following examples is a logical individual who wants the DLC and also has self-control over their spending:
You might want to buy it early because around the time of release is when there will be the most discussion on the game. If you want to engage in that discussion, it helps to have played the game, not to mention you can enter conversations about it, either online or with friends, without having to worry about someone spoiling major plot points for you. On the other hand, if you have polite friends, then they may try to pressure you into playing it so they can talk about the game without having to worry about spoiling major plot points for you.
If a game is centered around multiplayer, then buying early also has the advantage of getting a head start on developing skills for the game (though at this point, a lot of games like BF and CoD really don't change enough to warrant an early start). There also is the appeal of developing your own way of play rather than coming in after everyone else has already found out the best ways to play.
In a much less likely scenario, some actually understand how the business works and how important sales are when they occur close to the release date. If they feel like giving the greatest support they can to a developer they like, then they may just buy at release. If the game is good, they can give further support by buying the DLC separately.
Finally, there are just simply games out there worth the $50-60 that they are on release. One reason I enjoy the RPG genre so much is because quality is really the only determining factor in if it is worth the release price, as length is generally not an issue. A good multiplayer game can almost always pull this off, given that they have a near limitless length. Of course, all this depends on what a person deems worthy of $50-60.