Google Play is a monopoly in practice, but it's not like other app stores don't exist and might even be preinstalled. Samsung's store is baked into their phones. AOSP forked firmwares (Kindle, Baidu, LineageOS, etc.) might not have Google apps at all. But even on Google android phones, Amazon Appstore, Samsung Galaxy Store, F-Droid etc. can be sideloaded. Even apps like Netflix are turning into app stores to give away stuff with a subscription. Any APK at all is available to sideload assuming someone wants to.

So it's a defacto monopoly and perhaps it is intended that way by Google knowing few people will bother to install anything else even when the door is open to do it. Is it any different from most other platforms though? Apple is completely shut. So are XBox, Nintendo & Playstation. Even Windows has a preinstalled appstore that Microsoft is pushing hard with things like Windows S ("safe mode") to stop people from escaping.

As for Epic, I think they're just pissed that Google wanted a quid pro cut of the profits. They feigning shock that in return for putting Fortnite in front of a lot of eyeballs, promoting it, facilitating millions of download/installation/updates that Google would want something in return. Epic demonstrated they could sideload Fortnite via an APK downloaded from their store so they can pick their poison - distribute a game entirely for themselves, or do it via a store subject to the terms and conditions.