The simple answer is because it’s easier.
Someone with no background who becomes interested in Torah approaches it from a completely different angle than someone who grew up frum and begins to question.
The former is a much easier person to satisfy. If you’re discussing hashkafa the exchange boils down to this: s/he asks “what do Jews believe.” Hopefully you have what to answer.
With someone who grew up frum, it’s different. The questions are more complicated. Not all of them have easy answers and not all of them have answers at all. Because a frum kid with questions — say a mesivta bochur named Itche — does not ask “what do Jews believe?” He starts with “why has no one told me that Jews believe X if all the rishonim consider it so important?” and it only goes downhill from there. Quite often the person asked barely knows what “X” is. Then — if he doesn’t get disgusted and wander off looking for someone competent elsewhere (which many do) — he moves on to learning yesodos ha-emuna b’iyun, and when you learn b’iyun you have kushyos and stiros. To do “kiruv krovim” you have to be able to learn hashkafa b’iyun and know Yesodei Torah not as well as you know To’en v’nit’an — but as well as your Rosh Yeshiva knows to’en v’nit’an. Having been on the student side of that kind of learning, I know that I have met exactly two people in my life who are qualified to do that and only one of them is actually “in kiruv.”