This isn't only true for video games, but is also true for movies and music.
Since when do first day or week sales determine the overall success of an item? Spiderman three may have been the highest grossing movie in an opening week ever but it is is FAR from being the overall highest grossing movie ever. Halo 2 may have been the biggest selling game in opening day ever but in the end GTA SA sold almost two times as much during the same overall time period.

 So why did we start making first day and week sales the indicator of how successful a certain product is? Shouldn't overall sales be the truth?

People want instant gratafication. I think is always nice to know your product got out of the gate doing well then hoping it has legs or does well later. I think is why people say they are dissapointed with initial sales because they can't predict the future to know if it will pick up later on.

Who said they were ever an indicator of how successful anything is? You're having an imaginary conversation with somebody who never started it. Stop being a presumptuous moron.

As for halo 3, who wouldn't want to brag when they break a bunch of video game and entertainment records in multiple countries? I"m pretty sure if you sell 3 million + copies in europe + america on the first day that it's a sign of success too genius.

Except I never hear news media reporting when GTA SA passes 15 million sold. Yet it's all over the airs when a game sells 2+ million in a day and winds up with 8 million?

 First weekend results became important for the likes of movies when competition within the marketplace meant that movies rarely had "long legs", that is they were making a greater percentage of their income in the first or second weekend. As a result movies became more heavily marketed for the first weekend to maximise income before competition took away its sales in the 2nd, 3rd, 4th etc weekends.

 This trend exists to some extent in videogames. Games in Japan for example tend to not, on average, have long legs and therefore first week sales are hugely important to a games success. At the same time there is a far greater capacity for videogames to sell well even into their second or third years of release than there is for films. With films there are a limited number of screens and competition for those screens becomes more hotly contested with each passing year. Furthermore its desirable for films to move to DVD quickly to maximse sales in that format so they will not stay in the cinema that long, again increasing the importance of first week sales.

 Ultimately the videogame market is rather dissimilar to the film market in this regard. I believe everyone considers final sales as the true measure of a games success, only a "straw man" suggests otherwise. The same applies to films, of course, but first week sales are often incredibly important for that market given the time restraints that exist to make money in that format. 

 I am always amazed that people can get indignant over innocuous subjects such as market trends that are industry wide. The comment you jumped on didn't bash your sacred Halo 3 so relax.

As for movies, theatres get a larger portion of the ticket sale in later weeks so it is advantageous to the studios to collect fast.

 Video games don't suffer from the competition for screens but they are always elbowing for room on the endcap or by the register. If a game starts huge it gets free media attention and prime shelf location. Also it beats the resellers to the punch. That is a big issue in Japan. The extra nudging from marketing can get people to buy who may have passed once a round of negative reviews got to them.

For long term success it takes an unusually good word of mouth and just takes addictive gameplay and some luck. So game copanies are going with results they can buy.

I dont care ... I allways await the 1 month overall sales , before I can see an overall lifetime ... tough I like to guess a lot :P