Cable News Clips

I can imagine a lot of reasons:
* Victims may be reluctant to report crimes, if they or family might be undocumented or perceived to be so. There may be language barriers. The victims may not know what rights they actually have.
* Imposters are able to easily disguise themselves, simply by dressing in the hodgepodge of tactical gear that official ICE thugs wear. Even video evidence doesn't show a lot of clues.
* Even when local police want to track down actual ICE abductions, ICE has been reluctant to share info, or denied outright they were in the area.
* At a macro scale, especially in red state areas, the existence of imposters fulfills an actual goal of the Trump administration: to strike fear into the targeted communities.
* In cases where imposters have been caught, it seems to me that they've been really clueless, or violent to the point where police follow up as a public safety matter (for instance the attempted rape in an apartment lobby, caught on security video).

Under-reporting would likely be a major factor. The fact that there are so many turning up in the most cursory searches implies to me that those are the tip of the iceberg. Hard to prove, of course, yet impossible to disprove.