Why do bar mitzva boys publicly read from the Torah for their bar mitzva? Where does the minhag come from?
This tradition is actually becoming less common in the more "yeshivish" communities. The idea is that the enormous amount of time and effort spent memorizing the leining could be better spent learning gemara. This is probably true, but on the other hand, if you learn how to lein instead of just memorizing, that is also a very valuable skill and benefits the community greatly. Perhaps a compromise is to have the B"M boy lein just an aliya or two and also make a siyum on a masechta.
But in the end, it is really the peer pressure of the other 13-yr-old boys in the community that will determine what any individual boy chooses to do, much as it will be for the rest of their lives.
I once heard from Rabbi Nissan Kaplan that he did all mitzvos before then except for teaching torah to the masses. this is why he gives a drasha on the bar mitzva.
maybe the torah reading is similar idea. and also, maybe, to put him in the spot light and give him some honor, like a chatan to make the occasion a great simcha for him