The same exact anchor repeated everywhere becomes a problem because it creates an unnatural footprint, and that footprint flattens the signal the anchor is supposed to carry. When identical phrasing shows up over and over, it stops looking like the byproduct of independent choices and starts looking like a pattern someone engineered. The mechanism differs by where the links live, but in both cases repetition replaces a varied, informative signal with a monotone one that conveys less and risks looking deliberate.
For backlinks, the footprint reads as manipulation. Links earned organically come from many authors who describe a page in their own words, so a profile dominated by one repeated exact-match anchor does not look like something a crowd produced. It looks coordinated, which is precisely the pattern link-spam systems are tuned to detect. The repetition itself, not any single anchor, is what trips the signal, because the uniformity is the tell that the links were placed rather than given.
Internally the issue is monotony rather than a manipulation flag, but it still degrades the signal. When every internal link to a page uses the identical phrase, you give Google less contextual variety to work with, and the linking starts to read as templated rather than written for readers. Varied anchors let each link describe the destination in terms that fit its surrounding context, which reinforces relevance from several angles. Identical anchors collapse all that potential context into one repeated string, diluting the contextual relevance that varied phrasing would have built.
Vary your anchors so each one fits its context. Internally, describe the target in language that suits the page it sits on rather than pasting one fixed phrase everywhere, and externally, let links accumulate the diverse, branded, and descriptive phrasing that genuine linking produces. The goal is a spread that looks like many independent decisions, because that is both the more useful signal and the one that does not read as a pattern.