None of those look like Boba or Jango because of the time period. The first? Ancient Mandalorians, thousands of years before ANH. The second? Neo-Crusaders (which dont really have a Boba/Jango feel; rather, they have an Imperial Guard look), about 4,000 years before ANH. The third? Death Watch, a group that splintered from the true Mandalorians, and fashioned their own helmets so as not to be associated with the true Mandalorians. Even then, their gauntlets are virtually the same as Boba's.
Sure, they dont all have to be the same... but they dont all have to be different in a glaringly obvious way, either. Making a Mandalorian costume, and having others not mistake it for Master Chief or a Cylon, relies heavily upon symbolism. After all, you make this costume to show it off, right? What good is it to show off a Mandalorian costume if nobody has any idea what you are? There's nothing worse than walking around in a costume from Star Wars and having someone ask if you're from Battlestar Galactica or Farscape. In order to avoid this, you'd want your custom Mandalorian to represent the well-known Mandos in the movies, which people will then recognize.
Now, why are the gauntlets usually the same? Simple: it's cost-effective. It's difficult for amateurs to create completely custom gauntlets, especially if said-amateur is in college, with tuition costs and rent. Not to mention the hundreds of dollars that go into buying books for class. People like that (myself included) do not have the spare cash to buy clay and create molds, buy the material and assemble a vaccuuform table, and then buy the plastic on top of it all. Then they have to find the time to make it; not easy for a full-time college student who also has a part-time job. They'd have to do all of this just to get a quality pair of unique gauntlets when it'd be cheaper and more convenient just to purchase a normal pair (which will most likely be of better quality).
