There are phones nowadays with 12 gigs of RAM, but they'll cost you well over a grand. The majority of Android devices have much less memory — I'd wager most have less than 4 GB. And with the system taking up around 2 GB, that leaves user-installed apps little room to breathe.
One of Android's biggest strengths relative to iOS is how simple it is to sideload apps that aren't on the official app store. Rather than having to sign IPA files or tell your phone you trust a developer every two weeks, you can just enable a setting and be done with it.