I am playing this series for the first time (Gran Turismo 5) and confused - the opponents are like drones, they offer no challenge at all! Was it like that in the previous games too? All you need to win is to have a more powerful car than the drones. I have played it for a few hours now, it has it's pros and cons, but the racing itself simply isn't fun.

Because it is the best racing simulator ever created? I have played the vast majority of racing games out there and nothing comes close to GT. I also wager you have not beat GT at the higher levels if you think having a better car is all you need

Because cars and stuff.

The game is arguably more about collecting cars than actual racing and it does that very well. Plus now with online racing you can avoid racing against CPUs for the most part. You also have to realize that when Gran Turismo originally came out there was no game like it when it came to content and quality.

The Real Driving Simulator™

I played GT3 and GT4 and never enjoyed them that much. GT is rather about cars than racing.

Because back in 1997 it was pretty much without peer. Back then racing games were fully features with 3 tracks and a few cars. So suddenly having so many courses and hundreds of cars was a WTF!? moment.

The visuals were also pretty damn stunning by console standards back then too. The N64 was a joke for racing games, the Sega Saturn was already dead. So it really found a market, especially in Europe.

Far from true. There are plenty of other games that simulate racing much better. Road cars with unrealistic handling racing in made-up tracks on made-up championships with made-up rules on a made-up career with made-up money is not that simulating.

Name other good racing simulator at the time it was released

Because it's the only racing simulator that does its homework on racing simulation. Something people tend to forget is simulation means it is trying to be like the real thing. Other racing games can be argued as more fun (that's all opinion and conjecture), but GT is the most realistic. Period.

That's why it's popular. Because racing enthusiasts look for a real racing game and find GT feels like a real car and not like what people think racing is like.