There have been a number of alternative calendars. And in fact although the Georgian calendar has become the dominant calendar across the globe several others are in use like the Islamic calendar where the current year is 1437 AH, or the Hebrew calendar where the current year is AM 5776, or the Chinese calendar where the current year is 4713.

One thing you may note about all those calendars is that they all eventually would run down to 0. The Islamic calendar counts back to Muhammad. The Gregorian calendar counts back to Jesus. The Hebrew calendar counts back to the purported creation of the universe (but unfortunately they got that wrong by a few billion years). Any system that counts back to a specific start date will eventually have a AD/BC split, after event / before event. Even literary and fandom calendars tend to use this; the Star Wars calendar typically uses BY and AY (Before Yavin/After Yavin) making the events of the original Star Wars year zero.

The only way to get around that is start from the Big Bang. Unfortunately we don't know exactly when that was. The Current WMPA estimate is 13.772 billion years ago plus or minus 59 million years. So that's not terribly precise. Besides using 13,772,123,456 as the year field is a bit unwieldy. A lot of people will already shorten 1992 to simply 92, when they think the century is understood by context.

We could also get around it with an entirely backward counting system. A lot of cultures use that. In such a system this year is always year 0 (or year 1) and they count back. In such a system December 7th 75 years ago is a date that will live in infamy, until next year when it'll be December 7th 76 years ago. Once writing, and especially historical writing becomes common, such systems tend to disappear, as a movable reference point is difficult to maintain.

Another common system is to use regnal years and count from when the king was enthroned. Right now we are living in 57 Elizabeth II. The Roman republic had a modified version of this where the years were referred to by the names of the consuls. With such a calendar you theoretically never have "before" years, you just restart the count with each new monarch. In reality of course you eventually do get to pre-monarch years. It's also a bit unwieldy, quick how many years difference and in which direction between 3 Edward II and 12 Elizabeth I?

So there are alternatives, and several have been used by different cultures historically. But they all come with problems of their own.

Are you asking why we still use this system today, or are you asking how it came around in the first place?

We still use the system today, although there are other base years as well, because, ultimately, we have to start somewhere. We don't have precise dates for historical events and we need to decide a starting point, we can't just say "We are in the year 2,548,387 of human history" because, one, its unwieldy, two, we don't have precise dates. So any calendar year is ultimately arbitrary. While we could pick another one, people already know this one so we stick to it. Also, historians have somewhat moved away from BC/AD, we now use BCE (Before Common Era) and CE (Common Era). While they refer to the same years, they're secular.