Because weights move with prices, a cap-weighted index automatically stays in line as values change; the only trades needed are for additions, deletions and share-count changes. That self-maintaining property is what makes it cheap to replicate.
The consequence is that the index holds more of whatever has risen. When a handful of names becomes a large share of the index, an investor who thought they held a broad market in fact holds a concentrated bet. There are historical periods where the top ten names exceeded a quarter of major benchmarks.
This is neither good nor bad on its own: cap weighting delivers the actual aggregate market return, and any deviation from it is a bet that someone else must take the other side of. See equal-weighted-index for the main alternative and concentration-risk for the exposure it creates.
Related: index-construction, equal-weighted-index, free-float-adjustment, concentration-risk, index-fund, size-factor