f-droid doesn't have payments, Amazon is yet another faceless mega-corp that has had its fair share of sellers accusing them of various shady things.
Creating a Play Store alternative is hard for many reasons:
1) finance: the more countries you operate in, the messier it becomes. International taxes are one hard mess, KYC/AML regulations differ between countries, and to top it off there is the whole "international sanctions" issue especially regarding Iran (where it's fine and explicitly encouraged by the EU to do business with Iran, but any entity that has US exposure exposes themselves to liability in the US for violations).
2) vetting of apps against a constant onslaught of spam, malware, copyright violations: f-droid has it a bit easier since they require all apps be open source, but a commercial, widely used alternative will have to run static analysis, dynamic analysis (to catch runtime exploit attempts) and manual testing. All of this is expensive and requires expert knowledge of Android as well as IT security.
3) Implementation and hosting: an app store worth its name has a lot of binary assets to distribute to users (and again, you have to avoid getting into trouble with people abusing your service to spread illegal content, because there will be such cases rather sooner than later), the store itself has to be implemented, regularly adapted to account for changes in the Android core, you definitely want a focus on security to avoid some hacker distributing malware to all your users with a push...
4) Customer and developer support: it's a well-known meme that FB/Amazon/Twitter/Google are almost impossible to reach for ordinary people without raising a shitstorm on Hacker News or a well-funded lawyer team... but the key thing is, support is expensive to run.