Easy answer why... because you have to provide more when using the API... eg. by setting search grounding with Google APIs. The UIs are agent systems which will figure out if they need to use something like a search tool, APIs require you to set this manually.
did you read it?
do you understand how statistical significance works?
the literal hundreds of queries all outputting no reddit citations (Perplexity and Claude still showing this pattern from what I've tested today) does not follow from your "easy answer."
but, by all means, i'm sure you've got it figured out so, yea...don't bother with the research.