Because basically every other game has damage. I'm sick of the it wasen't in previous games excuse, if they followed this rule we wouldn't have the following.
Online
HD Graphics
more tracks & and cars
as well as no cars past the 2006 model year.
Well, GT4 was going to have online play, HD graphics are just standard in all games now (name one PS3 release that was SD only

), GT has almost always increased the number of cars and tracks (I think 2 had more cars than 3, though I'm not sure), and the last point... Yeah.
Damage is a slightly different kettle of fish. I agree the 'previous games' excuse is rediculous, but to get damage implemented they need to get agreements from all the manufacturers, and you have to remember this is being marketed as the next best thing to actually driving the car, a claim that is widely believed to be true, so they're less likely to be ok with wreckages than they are in, say, NFS. Also the hundreds of cars they have all have to be modeled differently to how they have been so far, with individual parts and such, which would up the processor use and polygon count and whatnot. I think if damage were to be implemented you might even see less than 16 cars on track, and if it came down to one or the other, I'd take more cars over damage.
The whole point of making a new game is to include new things and improve on what's been done in previous versions. Damage isn't a gimmic as you already know it is in real life.
It may not be a gimmick, but it doesn't really add a lot to the gameplay, at least not to the extent that most people seem to believe it would. It wouldn't deter online punters, they'd just love it even more, and if you drive properly you wouldn't even notice whether or not damage was implemented. For that reason alone, I don't think its such a big deal. There's a pretty big list of things I personally want to see included before damage, though I must admit, a properly designed damage system would be nice, just not essential for me.
Also they cater to the "idiot journalists" because most of GT5 sales will be from the casual gamer which unfortunately relies on their reviews. If one of the bigger name sites gives it a bad review trust me sales will be affected.
GT5 will be a waste of PD's time and money... GT5 is going to have damage or it just wont sell.
I'm sorry, but I disagree here.
Look at these examples:
Everyone knew MGS4 was going to be a must-buy after all the trailers and previous games, some were disappointed but it still sold a huge number of copies.
Killzone wasn't spectacular, but on the strength of the 2005 trailer (which generated a lot of controversy because no one believed it would be THAT good) and the demo, Killzone 2 sold well.
Call of Duty: World at War, even though pretty much EVERYONE moaned it was going back to WWII and the reviews said it wasn't as good as CoD4, it still sold well.
Resident Evil 5, despite not being a survival horror, sold very well, possibly thanks to RE4 and all the good reviews it got...
I could go on, but I think you can see what I'm trying to say. Based on the fact that the DEMO of GT5 was platinum before it was released (despite being less than 1/6th of what GT5 might offer for half to a third of the price of a full game), the huge communities there are online, the fact that you rarely see any Japanese gamers on these forums and they're probably GT5's biggest audience and finally the fact that, if Amar is right, GT5 will have 600 or so cars and 80+ tracks all either at or above the quality we've seen in Prologue, you've got a surefire hit on your hands.
I mean, GameSpot gave Prologue 7.5 out of ten and listed the following negatives- "Not nearly enough tracks Only a prologue to the real thing Online suffers lag at times No option to upgrade vehicle parts". Look at how many of those will be addressed (i.e. all, possibly except the lag) with GT5 on top of all the other stuff it's going to bring to the table. And besides, even if it still got 7.5, Prologue has sold well enough to be counted as a success by itself.
Oh and while this isn't related to your post, I really don't believe people on this forum who say they won't buy GT5 if it doesn't have damage. We all know everyone on this forum (well, most) will buy GT5 as soon as it comes out, even if only to get their hands on private race lobbies.
And to contribute to the topic, the only thing I don't want to see is wallriding allowed to continue unpunished. There must be something they can do about that... Oh wait, I've got it- Damage!
Sorry, that was a pretty epic post, I'll go back to my usual lurking now.