It'a not the role of Google to disallow phishing sites (as a browser) just like it's not the role of the ISP.
Make it hookable so people can chose their own phising protection service.
Phishing protection is mostly needed for people who have no clear concept of phishing or technicalities. They just want to do things on the internet, like social media, they don't care about things behind the scenes, that's boring uncool nerd stuff.
Chrome lets you override and proceed to the site. The problem for the small business is that a large fraction of their customers see the scary red warning page.
You just don't want to give control over the blocking blacklist/whitelist to a single entity, even less so to a huge powerful one, possibly in a country other than your own (which e.g. forces their foreign policy dictums to your blacklist), and even less so the one that already makes your browser, that should be a totally neutral conduit.
But also, it would leave the people most vulnerable to phishing unprotected, namely those not tech-savvy enough to install a phishing protection service. Most internet users don't even have ad-blockers.
Businesses become safer, but more regular people will get phished.