Start by looking how the Ramchal defines yir'ah. Mesilas Yesharim ch. 24 distinguishes between fear of punishment and yir'as Shamayim -- fear of [the One in] heaven. Then he identifies two aspects of the latter: yir'as haRomemus -- awe of G-d's Grandeur, and yir'as hacheit -- fear of sin, which comes from that awe. He writes that the default concept of yir'ah without a qualifier is yir'as hacheit. To explain how it differs from fear of punishment, yir'as hacheit is fear of doing the wrong thing because it's the wrong thing. Someone with yir'as ha'onesh is afraid of being personally hurt, something with yir'as hacheit is afraid of defying what G-d wants as an end in itself.
Someone without yir'as hacheit, who has no fear of doing something that defies Hashem's Plan for their life and for creation, will never get beyond focusing on their physical needs, and will never learn to delay beyond immediate gratification to find real meaning. Such people will never grow beyond operating on the animal level.
Rav Chaim Volozhiner writes (Nefesh haChaim 4:5):
According to the measure of the “silo” of yir’ah that the person prepared for himself, it is by that same measure that the “grain” of Torah will be able to enter, guarded and fulfilled within him, according to what the silo can hold.
It is [like] a father who divides grain for his sons. He divides it out and gives each one a measure of grain to match what the son’s silo can hold, which he [the son] prepared beforehand. For even if the father wishes and his hand is open to give him more, the son cannot receive more since his silo is not big enough to hold more. So too the father cannot now give him more. And if the son did not prepare even a small silo, then also the father can not give him anything at all – for he has no guarded place where it will remain with him.
So too Hashem, may His name be blessed: His “Hand” is open, as it were, to constantly bestow every person according to his reward with much wisdom and extra understanding – when it will be preserved by them and will be tied onto the slate of their hearts. Everything [is given] according to the volume of one’s “silo.” And if a person does not prepare even a small silo, which is that he does not, heaven forbid, have within him any yir’ah whatsoever for Him, may He be blessed, so too He, may He be blessed, will not bestow any wisdom at all, since it will not be preserved by him. For his Torah would become disgusting, heaven forbid, as our Rabbis, whose memories are a blessing, said. It is about this that the verse says, “the beginning of wisdom is yir’as Hashem."
Someone who has no drive to pursue higher goals may have free will in principle, he lacks the countervailing drive to choose the spiritual path over the animal one. He doesn't speak the Torah's language.