Because such bugs were predominantly associated with then-new platform features.

As a web developer myself, I appreciate the frustration with Safari's flexbox bugs of a decade ago and viewport bugs more recently. I also remember being endlessly frustrated by Chrome bugs too, like maddening scroll anchoring behaviours, subpixel rounding inconsistencies, and position:fixed bugs which were broken for so long than the bugs became the de-facto standard which other browsers had to implement. All browsers have bugs. To suggest that Safari was uniquely bad is to view history with Chrome-tinted glasses.