I suspect this is very much a learned habit for INTJs. That tertiary Fi, if reasonably healthy at all, wants to help others just as much as an xNFP. What happens, though, is that eventually we note that others are taking advantage of us and feel entitled to the help, or are ungrateful for the help, or otherwise treat the INTJ with disrespect or disdain in spite of the help.
I think this makes INTJs shield that part of ourselves with a cold, unemotional exterior, to prevent being asked for help. And when that help is requested, we really have no problem just saying, "No," no matter how rude it may sound, as long as we suspect that it isn't a genuine need.
As Oro notes, we gradually become older and more experienced, both with establishing boundaries and determining the motives of others, and we thus gradually become more open and helpful and compassionate when the need arises.
Definitely a learned habit. For myself, it was because the less people expected from me emotionally, the easier it was to stay in-sync with my own drumbeat. By not saying anything, you don't have to negotiate what has never been up for public debate in the first place. I have no problems with being upfront about Te-related skills, since I know this is something I can deliver consistently under the weight of enormous outside pressure, or change if I need to.
My Fi is rather unpredictable and flighty, so you're absolutely right about Fi needing maturation and experience before it can truly be open to others. One of my main obstacles was not drawing enough personal boundaries --- I seem to sacrifice my needs for the needs of those closest to me (like Uytuun). Actually, it was ridiculous to the point where an INFJ friend had to forcibly anchor me back down to reason, step by step. The more I was able to say, "No, I can't go through with this. It requires abilities beyond what I'm willing to do.", the better I was about sharing my sentiments. So, if I ever happen to mention that I enjoy helping people, I'd also make it very clear this occurs on my terms. This probably ties in with wanting to be competent at whichever field I choose, so being held to an unreachable standard is a burden. [That's my heighten sense of accountability talking.

]
Still, it's mostly an "oh, and, uh, by the way..." kind of thing.
I have no idea how applicable this is to other INTJs, so I'm mainly speaking for myself.
This is interesting. I think INTP's show compassion precisely in order to get along - not purely selfishly just because my Fe drives me to organize my surroundings according to the need for cohesion and inclusion - I like to see a group function and each person to have a role. It's adaptable to circumstance and I wear it outwardly.
Ah, I'd just normally let the cohesion and inclusion happen on its own, unless there is limited time. Then I would use Te to keep everyone on the same page. [
Ex: We're all after the same goal here. Let's stay focused on
solving problems we have now, instead of complicating them further. Personal differences aside, any ideas for improvements?] Or I'd use my Fi in an ENFP-like manner to inspire others to find their voice, and tie those similarities together into the group's objective.
Only when the situation becomes FUBAR, I'll rely on Fe.