While your surprise about the practice is understandable, you seem to have overstated the issue. We will explore the reasons for a distinction between married and single men regarding a tallit gadol, as we believe that it is important to avoid the outright rejection of established minhagim.
The Torah obligation to wear tzitzit exists only when one is wearing a four-cornered garment, but the Shulchan Aruch writes that it is important to create a situation in which one can perform the mitzva.[4] Although every individual relevant garment that is worn creates a mitzva that it have its own set of tzitzit, one receives credit for the mitzva by wearing a single properly prepared tallit katan.[5] Thus, it is incorrect to characterize one who wears tallit katan but does not wear a tallit as one who is avoiding a mitzva d’orayta. If a tallit katan did not suffice for the mitzva of tzitzit, we would likely wear a tallit all day.
Why do we wear a tallit when davening (Shacharit), considering that we are already wearing tzitzit? One issue is the concern that our tallit katan garments are too small for the mitzva of tzitzit to apply to them.[6] Failure to fulfill the mitzva of tzitzit might be of particular concern during Shacharit, when we recite the Torah section in which the mitzva is stated (the third section of Kri’at Shema).[7] The gemara[8] writes that one who recites Kri’at Shema[9] without wearing tefillin is like one who bears false testimony. The same might be said about one who recites it without tzitzit, thus strengthening the resolve of those who are careful to wear clearly kosher tzitzit (i.e. a tallit) at that time.[10] However, Tosafot[11] believes that this admonition is not applicable to reciting Kri’at Shema without tzitzit. The distinction they make is that one is not fully required to wear a four-cornered garment and thus have tzitzit, whereas he is required to don tefillin. While this is the subject of significant debate,[12] the concern that ordinary tzitzit are perhaps too small is not strong enough to countermand the minhag of single men not to wear a tallit.
The Maharil[13] is the classical source who cites the minhag for single men not to cloak themselves in tzitzit. Indeed, important poskim (including the Mishna Berura[14]) have difficulty justifying the minhag to deprive them of the mitzva. However, the dissenters seem to have understood the Maharil as saying that single men did not wear tzitzit at all, not as referring to our present minhag that they wear tzitzit but do not wear an additional tallit for Shacharit. Many understand the Maharil as referring to the present day minhag and have no problem with it.[15]
The question remains why one would want to deprive unmarried men of the advantages of wearing a tallit for Shacharit. (We will leave kabblastic reasons to others.) While the Maharil does mention the juxtaposition of the p’sukim on tzitzit and marriage that you mention, that hardly qualifies as a satisfying source without a specific reason for it. The Divrei Yatziv[16] posits that since tzitzit symbolize the need to observe all mitzvot and a single man has not fulfilled the mitzva of getting married, withholding the tallit encourages him to marry as soon as possible. We will suggest another simple reason. The Radvaz[17] posits that one should cover his head with a tallit during davening.[18] He says that if this were not so, we would not wear a tallit at all, given that we already have tzitzit on. At the same time, the Magen Avraham[19] writes that single men, and possibly even married men who are not talmidei chachamim, should not cover their heads with a tallit,[20] apparently to avoid appearing haughty. Based on the assumptions of these important poskim, it would be pointless for a single man to wear a tallit, thus explaining the minhag. In other words, the reason to wear a tallit is to cover one’s head, and unmarried men, according to the Magen Avraham, should not do that even if they do wear a tallit. While there are different practices regarding married men covering their heads during davening, the minhag for married men to wear a tallit is universally accepted. Thus, while we do not generally follow the Radvaz’s approach, his view may certainly explain the development of the minhag among single men.
In any case, when it comes to single men, both the minhag to wear a tallit and the one not to wear it are easily justifiable, and each person can follow his family minhag without compunction.