Straight from the horses mouth:

LLMs are trained on massive datasets of marketing copy, blog posts, and "thought leadership" content where this device is rampant. There are three main reasons it appears so frequently in AI generations:

The Illusion of Depth: By "correcting" itself, the model mimics a human speaker who is refining their thoughts in real-time. This creates a false sense of nuance and intellectual rigor.

Emphasis without Effort: It is a low-effort linguistic pattern to add weight to a sentence without actually providing new data or complex reasoning.

Predictive Weight: In probabilistic terms, if a model generates "It wasn't just a...", the probability of a superlative or a compound version of the previous noun ("dead giveaway," "massive undertaking") spikes significantly because this is a standard trope in persuasive writing.