Because there are those who say that IRCC doesnt take anonymous tips seriously.
I've not made this claim so not defending exactly, but:
...the context in which I've heard comments
like this are mostly about claims made after an applicant becomes a PR. (Esp the "relationship broke down and now it's clear that he/she was faking it" type).
And then the reality for IRCC is very different: demonstrating or proving a relationship is fraudulent after the applicants become PRs is much, much harder. Because the type of review is going to be different and the onus on IRCC to demonstrate that there has been misrepresentation.
During an application, however, IRCC can quite easily put the onus on the applicant(s) to demonstrate the relationship is genuine. And arguably IRCC has a duty to at least check - make the applicants respond by writing a letter for example.
So in the first case - after landing - I can absolutely understand why it might
seem IRCC isn't doing much (or anything) - if they inquire and there's no evidence that looks strong enough, they likely just drop it - and fairly quickly if they judge the likelihood low that they can get any evidence, it's costly after all. This might be completely unnoticeable to anyone on the outside.
Different circumstances entirely from IRCC perspective.
