I'm with kirmy, a lot of newcomers are on the server including me. Lots of them are not accustomed to the car and tracks and don't have a proper feeling for it in combination with their setup. Making setup changes mid-race means you won't finish for points. And making the races longer makes it a long sit with a car that doesn't feel right. Racing single-player doesn't work for me at least, since lap times will only improve to a certain point. But racing online you see the different lines you can take, where others brake, accelarate, turn, what their tires do etc... And maybe more important you are in some way "pressured" to go a little bit faster here and there when that car in the mirror gets bigger and bigger, or the one you view through your windshield.
You don't have this info ready for grasp or apparent pressure when racing single-player. Hot-lap replays are almost useless when it comes to racing or getting a feel for driving the car in a race.
Though I like they are a bit longer now, means you have to watch your tires a bit. But if you want really long races you should go somewhere else. Like more professional racing servers where the standards are a bit higher and people have the time to finish a long race. Most people don't have the time to sit there on end, they get disturbed while racing by roommates, family members, pets etc or just want to get a drink and take a break.