L'Shana Tova, everybody! My trip was cut short by two months due to my developing a respiratory problem during traveling. I have recovered and am home for the Jewish new year: 5776!
As to why Jews are SO different, I guess it depends on who you ask and your concept of what a Jew is.
First of all though many of share a common ancestry, we are not actually a race. Some Jews are not Semites.
As for those who do have "Jewish" DNA markers (as they are called, often referred to by some as merely "J"), we definitely do not look alike. Ashkenazi Jews look a lot more Caucasian than Sephardic Jews. I'm Sephardic, have a Spanish-sounding name, and many people never know I am Jewish until something causes it to come out into the open. Go visit Tel-Aviv and you will see Jewish people with blonde hair and blue eyes, red hair and green eyes, and a little more Sephardic Jews like me than what you will find in America or England. Jews are a collection of tribes that make up one larger tribe.
As for customs, it depends again on where you are from and what branch of Judaism you hail from. Sephardic customs are different from Ashkenazi customs. Reform Jews are very different from Orthodox Jews, and you would have a hard time pinning down a Post-denominational Jew. Some believe in G-d, others don't.
In the East it is customary for all peoples, regardless of religion or race, to pray with a head covering, to grow facial hair, to avoid pork, to have a national weekly holy day that forbids work, to worship using a language unique and exclusive to a national group. Also Easterners have been worshipping their deity (deities) longer than Westerners, so our theologies are far more complex, and many of us (Jews and Muslims) developed critical thinking during the Golden Age (when Christians were having their "Dark Ages"). This "Golden Age" of enlightenment opened us to advances in science, mathematics, even introducing logic into ancient religions. We are different in many of these areas because Westerners cut themselves off from us for a while and are still playing catch up.
Many of us have and do assimilate. I have had friends who for years did not know I was a Jew until a couple of years ago when Chanukah landed on Thanksgiving in the U.S.A. It startled a lot of people, let me tell you. I may talk a lot about being Jewish here, but only because religion is the main subject on so many threads. Outside of this site most people think I am Latino because of my name and the food me and my family eats.
So it depends on what you think a Jew is. Maybe you haven't been around enough of them. Only a small number dress like the Orthodox, whereas the majority of us would be hard to spot.