It's not arbitrary at all, it's for backwards compatibility. Previously the meter was defined in terms of the emission lines of krypton-86, which was a distance that we could measure most precisely with an interferometer. Before that it was based on the size of the earth, which could be measured with a astronomical calculations. And originally it was based on the length of a pendulum that produced a period of a 1 seconds, from which the length can be calculated using a scale, a reference mass and a clock (there was another definition, but this one was more precise I believe).
Each definition gives you something pretty close to what we now define as a meter, but the precision to which they could be measured at the time differed. The length of a "meter" was more or less the same distance in everybody's mind then as now, but if a 10000 scientists would sit down and perform experiments to actually calculate the exact length in 1700 versus, the mean of the values they reported would be about the same as it is today, but the statistical uncertainty would be much much much higher.
The redefining of the meter has typically occurred when some new process was invented that was more precise than the the previous method, for instance today measuring a laser in a vacuum is something like 1/3 the uncertainty than the old method using an interferometer. So the uncertainty is smaller, but crucially the mean value is still basically the same or very close. That's the reason for the weird 1/299792458 seconds, it's because we can define seconds very very precisely (from atomic clocks) and that's the amount of time it takes light in a vacuum to travel the same distance as the previous most precise known value for the length of a meter.
If it wasn't done this way, every time we invented a more precise method to measure distance and want to improve the precision of the meter it'd be like defining a whole new unit. Defining a foot as the distance light travels in a nanosecond is fine now, but if we discover an even more precise way to measure distance than a laser in a vacuum we'd end up in the same position, where a "foot" would be some strange fraction of a reference value that makes it work out to agree with the old most precise known value.
This sort of stupidly accurate measurement doesn't matter anymore on the day-to-day life scale as the length of a meter is known to within ~(10^-9)%, which is about a ten picometers. However, that means then that if you're fabricating silicon at the nanometer scale, the actual exact length of a nanometer is only known to within about 1% of a nanometer (if I did my math right, might be off by a factor of 10). That's much more significant.