I also played Gran Turismo 4 on the PS2 but I didn't spend that much time with it. The most hours I've put into a Gran Turismo game was the one on PS1. Gran Turismo 2 was my most played because it's what introduced me to the racing game simulation genre on the PlayStation.
When I was younger, I used to like split-screen when playing Dirt with friends but was a very confusing game mode at times. Personally, it isn't a huge deal for me although I agree to some extent that split-screen should have been available in Dirt 4.
I really enjoyed playing split-screen racing games with friends and family. Back then we only have few couch multiplayer games. I wish many of my friends and family are still into racing games. They are more interested in other game genres now.
Back in the days my brother and I used to be really competitive in Dirt. I used to play the original game for hours on end and so would he. Our races were usually extremely close but I had more wins under my belt against him. During some of our Dirt races, my brother tried to mess with my controller so he could beat me but it never worked - I still came out the victor. Nowadays, everybody is more into GTA and Fortnite and although I love racing in GTA, nothing compares to Dirt when I was growing up.
I've never played Dirt 4 so I don't think I have a right to criticize it. Although, if it doesn't have the splitscreen feature, which imo has to be the one thing that they shouldn't have left out. Then the devs did a terrible job on their part.
Some of the time it is fun to have splitscreen experience. Though not all the time it can be fun though. Some people just have their own ways to experience things and splitscreen makes things good enough.
When friends are around it is a very handy feature to have especially if your friend is into DIRT rally/racing games. Otherwise, Not much use for it and with the availability of the internet, most people opt to go the online route because you can play with more people.
I'm used to playing with splitscreen and maybe that is the reason why I'm into racing with the splitscreen feature on.
It comes with the territory though. Let's use GTA online as an example, the community is an absolute cesspool and yet the game is stronger than ever. It is a bi-product of huge gaming communities - especially competitive ones.