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Because it’s not really a line in the first place. Every country on Earth can choose its own time zone. If you’re on the opposite side of the Earth (latitude-wise) from London, England, which is where we have historically decided to put the “zero time zone” (also known as UTC), then in solar time (i.e. when the sun rises and sets and when it reaches its highest point) you’re about 12 hours away. But whether you choose to call that 12 hours ahead or 12 hours behind is arbitrary and up to you. Most countries choose time zones that are a whole number of hours away from UTC, so that it’s easier to calculate with, so if you’re in that part of the world, you can either choose to be in UTC-12 or UTC+12.
Let’s say it’s 6 AM in London on Monday. If you’re in UTC-12, that means you’re 12 hours behind, so in your time zone it is 6 PM on Sunday. If you’re in UTC+12, then for you it’s 6 PM on Monday. So these time zones differ in their calendar date by a whole day, even though the time on their clocks is the same. In between two countries that are in either time zone, there is an imaginary line where the date changes if you cross it. A “date line”, if you will.
Generally, countries that are just under 12 hours ahead of London in solar time will choose UTC+12, while countries that are just over 12 hours ahead (i.e. just under 12 hours behind) will choose UTC-12. But this isn’t a hard and fast rule. In solar time, the date line would be a straight line from the North to the South pole. But that perfect solar date line runs through the middle of some countries, and countries (especially smaller ones) don’t like to have different internal time zones if they can avoid it. So if a country mostly lies west of the “solar date line”, and only partly east of it, then they’ll likely opt to have the whole country use UTC+12 as its time zone, which means the actual date line now lies east of this country rather than running through it.
Even if a country lies completely on one side of the “solar date line”, they might still choose to put themselves on the other side in terms of their time zone – for instance because their neighboring country uses the other time zone. In fact, the reason doesn’t really matter: each country simply has the ability to choose its time zone however they like. If New Zealand decides it wants to be in the same time zone as Germany, that would be silly, but it’s their choice. Also, there’s no rule that says you can’t go beyond UTC-12 or UTC+12. Kiribati, for instance, uses UTC+13, which means their clocks show the same time as UTC-11, but their calendar days are not aligned. There’s even a chain of islands, called the *line islands*, which uses UTC+14.
In summary, the actual (*de facto*) international date line is just the result of a bunch of individual countries’ choices – choice that only roughly follow the solar time. That’s why the line zigs and zags, especially around the line islands where it makes a funny loop and doubles back on itself. There’s no reason why it even has to be an unbroken line. If Kenya decides tomorrow that it wants to be on a different calendar date from its neighbors, then that will create a “date enclave” and there will be no way to draw this on maps with a single unbroken date line.
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