As you might well know, Yii 1 was (initially) shipped as a standalone library that needed installation on the target environment, and from there you could use its CLI interface to create your web app. After that, the library would be sitting somewhere in your filesystem to be directly called by the web app upon loading it.

When Yii started, this was common practice; there wasn't a way to keep the code self-contained and you could easily get into several problems whenever you needed to ship the code to shared hosting environments (I'm looking at you Plesk/OpenBaseDir restriction).

Secondly, system-wide packages and dependencies were often restricting the developers to embrace new features and work around existing bugs, without even counting that these were (too) often overlooked. If you've been working on the web with PHP for quite a while, I'm pretty sure you've experienced the sense of lagging behind other big frameworks on the development scene (and not just in PHP-land...