How can you say its impossible? Improbably yeah but I don't think its that restrictive. In fact I think the quality is improving on the iPhone and you will see more expensive games on the top 20.
The games that do crack the top 20 above that price tend to be big franchise titles like Sonic or certain EA titles.
There is no doubt the quality is improving on the iphone. That's never been in dispute. There is a worrying problem however of prices being driven down which long term isn't a good thing on the business side. It's short term good for consumers but not long term. Iphone users are being over-indulged with too many good cheap games in this case which creates an incentive to not produce higher budget efforts unless they are extremly big franchises simply because the economics work out better in that situation.
Apple needs to separate out the 99 and $1.99 cent apps from bigger budget efforts because they tend to choke them out. Essentially the premium store content idea but not at the crazy price tag of $19.99 like it was rumoured to be earlier.