Ember actually has a pretty great onboarding experience these days. Ember-CLI's generators & addon allow you to create and deploy an app in ~5 minutes from `yarn install`, plus it comes with a "welcome page" embedded in the app to help you spin up. Their docs are also pretty great, and you have www.ember-twiddle.com to play around with Ember, too.
At the end of the day, the Ember team takes a lot of time to not just build things the right way, but also make it as painless as possible for teams to upgrade along the blessed path. This has been a learning process to understand how much more time that takes, and has caused some past announcements to feel like they weren't delivered on, when in fact they simply just took more time to get right.
Why would you want to build a bespoke React-based framework for every app you make? Why not simply have everything you need out of the box, plus the ability to easily integrate any npm library into your app via first-class build tooling that is miles easier to use than something clunky like Webpack?
Glimmer is smaller and faster than React, with TypeScript supported natively. This is a game-changer for people who are looking for a lightweight rendering library that ships with great tooling support and a larger framework ecosystem behind it.