I wouldn't use the word random, more like unexpected, and it just makes it more exciting.
Whichever team makes the fewest mistakes wins, it's just as much about mental toughness as skill, allows teams to surprise their enemies with unexpected tactics and tests them to see if they can react and adjust.
It's not just that, it's also maps. Some teams are going to be better on some maps than others, so having a bo3 will give you a lot more accurate count of who's better skilled overall, and not just better at a particular map.
Well that can lead to the same result as a best of one.
The first map is random-ish, and the team that's better at that map is going to win.
The losing team picks the next map, something they're generally better. They could lose it here or win it.
If the team that won the first match loses the second match, they get to pick a map they're better at and have a high chance of winning.
That first map plays the largest role in determining the overall winner. That's not saying teams can't come back and get two wins, but I would suggest that it's very uncommon. I'd have to go and check the past match videos to verify that however.