They could accomplish that by doing strictly objective technical QA. However, that is not even remotely how the major app stores operate. They engage in massive censorship to shield their own apps from superior competition, protect their business models, appease their business partners, and finally, to enforce their particular morality on the content that may be offered.
I think that generally developers miss the point of closed app stores. Yes, they restrict what you can sell on them. Yes, it costs money to submit apps to them but they also offer a sense of safety to consumers and a direct way in which you can market your apps to the most number of people.
Allowing developers to do what they want only works if every developer is of reasonable quality. Unfortunately this is not the case so someone must ensure that the apps that are available to consumers (who probably know nothing about security or computers in general) are of a certain standard in order to protect said users.
Sure. There are problems with the app store model. I'm not saying it is perfect. What I am saying is that from a consumer perspective (and lets be honest here that is the most important perspective) app stores offer a huge advantage.
Before app stores were common place the only way to find new software was either through word of mouth, advertising or Google searches. Advertising is notorious for giving consumers a false view of what software enables them to do so consumers (in my opinion rightly so) are skeptical of what they hear through that method of communication. Word of mouth is great but you need to keep your eye on tech sites / forums to get the most from it (which most consumers do not do) and Google searches often just reveal a small subset of the available options and there is no guarantee that what you find is the best option.
App stores fix all those issues. Consumers have one place they need to look to find all the apps that are available for their chosen platform. They know that the apps have gone through at least a minimal amount of vetting and they can see other user reviews of the apps.
From a developer perspective app stores also have a big advantage. They take care of distribution and software updates. Often setting up a website that you can sell your software on then advertising the website and making sure the website ranks highly in Google as well as having an update infrastructure can be an expensive undertaking, especially for indie devs. App stores take away the hassle for a 30% fee which if compared with the amount of money a developer would get from a normal distributor is actually a very low price.