Extensions for Chrome are available only through the Chrome Web Store to install, and no longer beyond. Google is taking this action for deceptive installation of malicious extensions to avoid.
Websites can now have self-extensions, offering, than, for example, via a pop-up to install. But that can also make sure that Chrome users accidentally add-ons to install those evil intentions. Later, one can still be forwarded to the Chrome Web Store to extensions via a safer detour to install.
Recently popped up as a kind of “inline extension”, which via a fake YouTube page was distributed. After the installation, were passwords stolen, and was the affected pc used to cryptomunten to mine. Nevertheless, it is the Chrome Web Store itself also is not totally free of this kind of practices, so it remains still a word of warning though.