Having a name in Japan is not as simple as it is in a place like America. First, there is actually an official list of possible Japanese names. Traditionally, to choose a name the new parents must visit a Shinto priest who specializes in astrology. The priest looks at when, where, and how the baby was born, and produces a list of auspicious possibilities. The parents then choose one they like. Even parents who do not go through this generally choose from among a commonly accepted cannon of names.
The other important aspect of Japanese names, and names in general, is that the kanji or words used to represent them do not have the same meaning that they do as a regular part of speech. The same is true in English, e.g. Chad the person vs. chad of Florida election fame. Because of that distinction between proper names and other parts of speech, certain strings of characters are only recognizable as names, and do not make sense any other way, and vice versa. Though it is easy for most foreigners to “convert” their names phonetically to kanji, these characters are often incomprehensible as names or anything else. It would be the equivalent of a person coming to America and calling themselves something like Emotion Monkeynoodle. That might be a good name for a band, but at first glance it simply doesn’t make sense. And unless you are a rock star, a Mr. or Ms. Monkeynoodle is going to have to suffer some confusion and discrimination with a name like that. Some foreigners in Japan opt instead to go by a Japanese name, but legally this is merely a nickname. To avoid this kind of confusion, the Japanese write all foreign names phonetically in katakana, thus marking them as non-Japanese names. The exception is when a foreigner becomes a naturalized Japanese citizen, at which point they are required to take and register a “normal” Japanese name.
Of course, Japan is certainly not the only country to strip down the names of foreigners to make them more palatable to the locals. How many people became a Smith or a Johnson on Ellis Island because the guard on duty couldn’t understand or spell names like Taszycki?