same reasons as above. Temporal decorrelation, low coherence, not enough fringe patterns, unwrapping errors, atmospheric disturbances, unsuitable perpendicular baselines…
Sentinel-1 is not the best base for DEM generation. Please have a look at these explanations
- considerations of @falahfakhri on DEM generation with S1: Phase to elevation - BLACK product - #20 by falahfakhri
- considerations of @qglaude on DEM generation with S1: Phase to elevation error - #5 by qglaude
- error sources of SAR interferometry: https://eo-college.org/resources/insar_errors/