Because GT3 is fun.
I've always looked at it like this:
GT3: Quality over quantity.
GT4: Quantity over quality.
To be honest, though, I can't see why anyone would recommend either of them (though GT3 is easier to understand) over the incredibly wide variety of amazing games that came out on the PS2.
LaBounti
I tried GT3 after 4 and can't go back to 3 because of the physics, car list ,track list and lack of full DFP support.
I bought GT3 a year and a half after GT4, and I was amazed how much better it was than GT4 was. If I had to choose only 1, I would choose GT4 simply because it has more cars. But nowadays I play GT3 far more than GT4. I suck at Enthusia, but I play that more than GT4 as well.
And GT4 physics? Dios mio!