It's good that most people are right handed
Apparently being right handed is caused by the left side of the brain being dominant. So, what would happen if neither side were dominant? We would probably develop a preference naturally for one hand. Half the people would go with left, half with right. Or even if it was a 50 50 chance that your right side of the brain was dominant in the first place, making half the people left handed from the start.
It's true that most things are designed for right handed people. Left handed people can learn to use them well, and won't have any problems. If it was half and half, there would be much more conflict about standards. A left handed and right handed model would have to be created for most things.
Because the number of left handed people is small compared to right handed people, left handed people just accept right hand made things. Obviously it is not always any more difficult to use them, but imagine now if most people were left handed. Would the clock go around the other way? The hot and cold tap? Would we read from right to left? Reading from right to left might sound silly right now but unless there is another logical explanation for that I'm sure it would have been possible if most people were left handed.
So what if it was half and half? It would cause problems from left handed people wanting it one way and right the other. We're not just talking about in a well civilised time like now, I mean from the start. I highly doubt we would be advanced as we are today.
But because right handed people have been the majority, left handed people have been brought up with the way right handed people saw best. Now a lot of left handed people can use their right hand better than right handed people can use their left hand, so in a way left handed people are advantaged.
The main point is that if it were half and half it would not be as easy to advance as humans.