I wouldn't quite phrase it as "because no one can see inside them or truly understand their unique person/situation." I would more say "because the solution to the situation has to be consistent with the individual in order to be effective." If the solution is externally imposed, you end up either acting in conflict with your values (big problem for Fi) or doing a half-hearted job (which is just a waste of time and energy).
At times the over-helpfulness of INFJs feels a bit like having a clumsy if well meaning bull in your own, personal china shop. "Well, why you don't just move this..." (crash) "...and then put this here..." (thump). "Isn't that better? I don't know why you didn't do that before! If you wouldn't be so resistant we could have gotten this done a long time ago!"
Then you see the INFJ's surprise as you peevishly escort them off your value premises. The funny thing is the INFJ may not realize that there were premises and that they crashed through a fence, front door and knocked over a table on their way to help (for your own good). Sometimes it can feel like the INFJ wants to improve you for selfish reasons: because it's like irritating grit for the INFJ otherwise.
Sometimes it seems like an ideal friendship to an INFP is like living next door to an acquaintance. You have an open invitation to ring the doorbell (call ahead, when possible, please) but if the other person isn't home or isn't answering, it's not big deal. When you are let in to an INFPs emotional space, you treat it as an honor and show you value the friendship by not touching things without invitation, because you assume they've already invested a lot of time and energy to get things into the shape they are in. Friendship to an INFJ seems more like moving into a commune. There is no personal emotional property once you've let them into your life. Everything is a potential fixer-upper.
All teasing aside, I tend to tell people I neither give or take advice well. When someone does actually want advice and explicitly asks for it, usually it's a matter of finding the solution they already know on some level. At most, it is making suggestions that are consistent with their values and wants that still meet the external requirements of the situation. Other than that, about the only other thing I feel I can add is my perceptions of the situation, especially if the person asking for advice isn't emotionally clued in already.
Anyway, I hope the INFJs don't mind the teasing. Don't know why this topic brings it out in me.