It's not easy to just hire a horde of people to work on an enormous, high-profile game like Gran Turismo. For one thing, individuals who are highly skilled at the sort of stuff you're required to do to make such a game, such as AI programming, sound engineering or 3D modelling, are very difficult to come by. Then there's the issue of actually getting all those new people to fit into their company. Literally. For all the size and scope their games encompass, Polyphony is actually kinda small, with only two offices in Japan to its name. Not to mention Kaz's potentially perfectionistic nature when it comes to hiring people...
In any case, Polyphony has actually expanded greatly since the release of GT5, swelling from over 140 employees to somewhere over 300, if my mind serves me correctly. And they're supposedly still hiring. So they surely know where part of the problem lies.
The stuff GTPlanet members have been requesting the most for GT6 (really just more cars and tracks and better AI and sound) isn't really something that requires more horsepower. Besides, the PS4 has been available for game developers to develop on for quite some time (probably somewhere around the release of GT5), so that avenue was already available to the company.