Recently I have heard people comparing learning gemara to a Phd program and saying that if people can learn nuclear physics in college and graduate school why not Torah.
IMHO, the answer is that there is much more to learn in Torah then other disciplines. The reason being, that these days there is so much knowledge no one even attempts to learn it all, everyone specializes in their own niche. Nuclear physicists for example, specialize in nuclear physics but know little or nothing about other branches of science. However, in Torah, a person is supposed to know kol hatorah kula. You aren't supposed to specialize, you are supposed to know it all. The gedolim throughout the generations until today have done just that. They know kol hatorah kula. Therefore, the amount of knowledge needed to get even a Phd is a fraction of the knowledge needed to master Shas and Poskim, Chumash, etc. You can ask a Gadol a question anywhere in Torah from Kodshim to taharos, to nezikin to zeraim and he can answer you. That is the equivalent of 1 person being the expert in chemistry, biology, physics, etc. That person doesn't exist in the secular world.
Let's think about a typical day for some who really wants to master Torah. Let's say you have 12 hours a day to learn (which is much more then the average Phd student will put in).
To learn 1 daf well with all the rishonim etc. on average let's say 4 hours.
Chazara on what you learned, at least 2 hours a day.
Mishnayos - 1 hour a day
Chumash, Nach 2 hours a day
Mishna Berura - 1 hour a day
Shulchan Aruch (Yoreh Deah, Choshen Mishpat, etc.) 1 hour a day
Miscellaneous - 1 hour a day
At this pace you will finish Shas in 8-9 years (Daf Yomi is 365 days a year, this schedule is clearly not, shabbos , yom tov, chol hamoed no one is learning 12 hours a day) and maybe remember some of it. In other words after almost 10 years of intense studying you will have finished Shas once and hopefully know Chumash and some halacha.
You will still not have learned any Talmud Yerushalmi, Zohar, every Sidrei Tahara in Hilchos Nidda, read all of R' Akiva Eigers or the Chasam Sofer's teshuvas, know all the Midrashim, etc. In other words, you would still have a long way to go to knowing kol hatorah kula.