Why is there not more modern resistance to the idea that the Talmud, written by rabbis, carries the same authority as the Torah, given by God at Mt. Sinai?

Let me begin by pointing out that your question is based on several assumptions or presuppositions. It presumes that the Torah was written by G-d, that the Talmud was written by rabbis, and that either or both have some level of authority. You are referencing a distinction between D'Oraita (from the Torah) and D'Rabbanan (from the Rabbis).

There have been those in history who rejected outright all material that was not D'Oraita. They were called the Karaites. They wanted to live specifically and only according to the Torah. As time and history moved on, and the world of the Torah was replaced by the world of the rabbis and then in turn by later realities, it was necessary for Judaism to face and adapt to the new reality and to deal with it. Failure to deal with the reality in which you live leads to extinction both in history and in evolution. The Karaites are effectively not a factor in Judaism today because they did not adapt. The rabbis found ways to stay true to the core of Judaism while adapting to the changing reality around them. 

To answer you more clearly, I will speak hypothetically from the position of the most ‘extreme’ Reform position. (I would point out to you that this is not the position of all in the Reform movement, nor do I believe that it speaks for a majority today [nor is it my own view]). This is more in line with the thinkers who formulated Classical Reform - itself an adaptation to a particular time and reality.

Viewing your question from that perspective, your question begs a question. Who says that the Torah was given by G-d at Mt. Sinai – is that not sacred myth, rather than fact? If that is the case, we must ask, resistance to what, for what purpose?
To elaborate, many people following the Classical Reform view within the Reform movement would accept the proposition that the Torah was written by people (prior to the time of the rabbis, far earlier than the Talmud), but is still of human origin, as is the Talmud.

Within that viewpoint, this is not a question that has much relevance. Classical Reform holds that we are not obliged to accept that the Torah was divinely written, nor that the Talmud is anything more than an attempt to further the social and legal system developed by people to provide order and stability. Why worry about the relative weight of the components of the system, or about the level of authority of one component in regard to another? Why would we ‘resist’ the value or applicability of the Talmud, when it is not seen as binding law or as carrying any legal authority for Reform Jews? Why be concerned about how much weight is given to the Talmud relative to the Torah, when the Torah is (itself) not viewed as being of divine origin, or having any special legal applicability? In fact, neither has any more or less weight for this purpose, according to this viewpoint.

Other responses from within the Reform ambit might well look more like those that you will receive from a Conservative or even an Orthodox perspective, accepting that Torah is divinely inspired, or divine in origin, and that Talmud is the combination of the Oral Law (oral Torah, divinely inspired or given) together with the interpretation of it, so it has similar weight and value to the Torah. It is only within that portion of the spectrum of Reform thought that your question has relevance. 

For those who see Torah (and even more so, Talmud) as having divine inspiration or origin, there would be no need or reason to question the authority of the Torah or Talmud, or to concern themselves with the distinctions overly much. Today, the distinction of D'Oraita and D'Rabbanan seems only to have applicability in some matters of fine interpretation of Halachah. There is can matter greatly, but for most it is not an everyday concern.

 

There are a couple of premises in what you write that I don't share. First, there has been resistance to the authority of the Talmud and to the rabbis who are cited in it from the days when those rabbis lived until today. For example, the Sadducees and the Karaites did not see them as authoritative. Similarly, the Reform movement today also does not see them as authoritative, and many in the Conservative movement are far more willing than I would be to change what they have said.

Second, even in the Orthodox community, the authority of the Talmud is different. The Bible does not include debates concerning its legal pronouncements. There are hundreds, if not thousands, of debates in the Talmud, and decisions have been reached over the centuries by later scholars as to which opinion to follow. But some of those decisions have still not been made and are at the root of different rabbinical or communal positions to this day. So in that sense, it is not as authoritative. In fact, in the discussions of why no one can argue with a final decision of the Talmud, the reason given is because later generations agreed that it was closer to the source of the tradition, and that therefore, we would not argue with it. But there is nothing in the Halachik process that precludes such an argument absent that agreement. Therefore, in traditional circles, where the Talmud provides settled law, it is seen as authoritative. On the other hand, the Bible is always seen as authoritative without any further analysis, discussion or historic process.