Different people have different "mindsets" when it comes to this program. I use the word program, because personally I dont see this as a "game". For me it's a great "toy" - something to toy with, like lego, ie something to tinker with.
Perhaps the older generation see it as this, and are not bothered about completing the "game" (me included) and the younger generation generally see it as a game to be completed and then shelved.
My stepson cannot understand in the least why Im stuck on GT5. Its the only thing I use the PS3 for and I have been immersed in it since Xmas. I am not interested in any other title.
Im loving the tinkering aspects of GT5 and the possibility to attack it in many different ways within the ongoing experience. For example, Im slowly building up my garage at the moment, just buying odd cars from the UCD and premium dealerships, putting them in storage. And for the racing side, Im working backwards through my garage in reverse order of PP testing out each car, tweaking tuning and upgrading each using the cashcow of the Supercar Challenge in the Online Seasonals to generate cash whilst im enjoying this. Marvellous. Im making notes about each car and marking them as fave's as appropriate.
Its an ongoing and very satisfying experience. Am I interested in getting to level 40 and getting ALL the cars? yes, sure, but not greatly :-) - its a completist by-product of the driving experience which is GT5.
Perhaps also being a fairly experience driver in the realworld helps to appreciate the subtleties of driving in GT5 and the differences between setups and vehicles better. Thus the lack of needing to "complete" the "game" become irrelevant in light of the magic of the driving itself.
I think with something as complete and complex in terms of in play variables is bound to hit different people differently psychologically depending on their maturity, demographics, age, experience, etc.
By default because of it's completeness in terms of basic driving physics, it CAN please such a wide range of people.
Using a realworld analogy... everyone LOVES the bumper cars/dodgems at the fair, but we all play a little differently; some guys will try and ram everyone, some guys just love cruisin' :-)