"I just don't see why they should combine it with GT" - If you'd ever played the original TT, you should know that GT4 and TT shared A LOT of the same resources. It's a lot more practical/efficient to combine the two. Plus, it's not necessarily a matter of "why should they?". For the people who like bikes, it would just be cool... almost like getting two games for the price of one. And it's addition would almost certainly be implemented in such a way as to not hinder the experience of those who don't like bikes. So I ask you: Why shouldn't they combine it with GT?
except this is a terrible business model, getting two games for the price of one is unheard of in the pay to play/DLC era, especially from PD/Sony where profit is a necessity for future development and continuation of the series.