Cost. TGC's models are not anywhere near as expensive as much Unity/Unreal stuff out there. I've even had people complain that things are too expensive NOW. It was a $6 script, but really, the gall of some people, calling that expensive, it's not as if artists actually make anywhere close to that amount either. If $6 is too much then go write your own is what I say, and I am guessing that to get anywhere close to the script I provided the user in question would have had to spend many hours getting it working, or look around for free ones to try to cobble together. Either way, $6 is not a lot to spend to get access straight away. Try buying Unity scripts for that price
It's the same with models, I've seen some crazy prices on Unreal's store, $30+ easily (liveable if you really want it, but obviously it doesn't take many of these to see your game budget going skyhigh) and some are much higher than that. Try this on GG's store and expect very low sales, unless it's a pack with lot's of content.
Finally performance. GG models are normally aimed more at performance (although not all, some are real hogs) Making models of the quality you say would probably slow down GG quite badly, as the only way to really increase quality we have is via bigger textures and higher polygons. There's no other tricks in GG's arsenal. Unfortunately, GG is aimed as an easy game maker and thus aimed mostly at beginners, and so you will always get beginner art served up as well on occasion. Stuff that I wouldn't even give away, has been put on the store in the past. High poly, awful textures (just stuck on, no real UV mapping as such), seam glitches you can see through etc. In honesty some of the official TGC stuff suffers from the last item.
It's an amusing Catch 22 situation that GG is aimed at beginners, but the only way to make a good game is to know what you are doing. Beginners are very unlikely to make anything decent with it, it takes dedication and work to get something half decent made in GG. Experience helps greatly as well, coming to GG from say Dark Basic, AGK, Game Maker Studio or any other programming language/system will greatly increase you chances of getting Game Guru to produce something other than "Catapult Shooter".
I agree in a way, I would like some quality models as well, often GG store ones look okay, but are not quite right. At least sometimes. There are some excellent models on the store amongst the less excellent ones. It's rare to see a AAA quality ones though, but as said GG would probably grind to a halt if you added to many showpiece models in. Also, I really wonder if they would sell at the prices they would undoubtedly command. Most people here are on a shoestring budget in the first place.
Finally, you do have the option of buying said media from the Unreal store and using it in GG (if the license allows it, I just wouldn't buy if not, as I consider media to be media, once you have paid for it you should be allowed to do as you want with it, bar sell it off as your own model or pack of models.) Yes, you probably would have to do a bit of conversion, but static items should be easy at least.
SPECS: Q6600 CPU. Nvidia 660GTX. 8 Gig Memory. Win 7.