At the time, Microsoft's hegemony over the web browser market was at its peak. Firefox was the only real competitor. If web pages checked the user agent, they probably only accounted for these two browsers. Anything else fell back to legacy behavior, disabling DHTML (later called Web 2.0). This made Gmail suck.

KHTML had almost no market share - it only ran on KDE, and most KDE users used Firefox. Nothing recognized its user agent string, so reporting "KHTML" would have been as ineffectual as reporting "Safari" or "WebKit".