Because you need to really appreciate/love cars and motoring to "get it".

GT is not an arcade racer.

GT 1 & 2, are still my favorites.

I have played racing sims before, my favorite is Viper Racing (PC, 1999) which is a viper sim that surprisingly holds up really well and even though it has only one car model and a handful of tracks it c*aps all over GT5 as far as enjoyment of racing and fun factor.

yea i'm gonna have to go ahead and disagree with you about the N64 having no good racing games.  

I didn't say it had "no good racing games" I said it was a joke for them. Compared with the PlayStation and even the Saturn, the N64 racing game lineup was awful.

I loved Mario Kart 64 and Diddy Kong Racing (but these are Kart racers which is a sub genre), Ridge Racer 64 and enjoyed Top Gear Rally, but off the top of my head, I can't really think of anything else worth bothering with on the N64 as far as racing games went. There was a reason when Burnout released on the GameCube, the Nintendo magazines went crazy for the fact that there was finally a good racer on a Nintendo system.

Oh I suppose there was also Wave Race, but again, not the sort of racing game we're referring to when talking about Gran Turismo.

I loved Gran Turismo 1-3, haven't played the series since then (well I played GT5 for like 10 minutes) I just don't like racing games in general much anymore.

Because it was a novelty having loads of different cars to drive and quite some race tracks back when the first one was released.

Nowadays GT has a lot more competition, games that in general have better physics despite not having that many cars, like Project CARS and Assetto Corsa, the latter which will release on both PS4 and Xbone next year.

A new GT will have to pick up a lot of steam if it wants to compete with the likes of these new race car simulators, not just in having a lot of cars, but also refine the gameplay in itself.

I still disagree, unless you only mean realistic simulation racers which you did not say.  Still Top Gear Rally 1 and 2, Ridge Racer, and F1 world championship 1 and 2 were all great games at the time in that category.  If you expand into futuristic or arcade type racers the N64 comes out pretty good.

For example, GTR FIA Racing was released at the same time GT4 was released and the former has motorsport cars (not road cars), a more realistic handling, real tracks, real championships and real rules. It's far more simulating than GT4.