There being no obvious available candidate and the difficulties with regards to timing (no preseason, tough fixture list, two games a week) may be the reason Frank is still in the job.
Frank has shown himself to be a good manager in the right circumstances, like almost all managers at this level.
To me it sometimes reaches a point where someone just have to go. We couldn't know we would improve when we sacked Conte, Mourinho, Pochettino etc. I'm not saying we're at that point now. But to me there comes a time when a manager just has to go, things have soured sufficiently, the circumstances are such that even though they are a good/great/whatever manager they're unable to do good work. Not saying we're at that point now, but some signs of us heading in that direction at the very least.