I think the bottom line is money.
If you are just staring out, finances are touchy and you just want to shoot and have fun, then a used rifle can be the answer.
You might luck out and find a fine rifle that someone is selling because they're financing a new build; or they're dropping out of the game.
Or the used gun might be off just a bit, and in the hands of a good gunsmith; a new barrel and some machine work might bring it in line.
If many cases, a used rifle is not necessarily going to get you into the top winning ranks, but it could provide you with a high "fun" factor.
Building a new rifle brings it's own set of challenges. Working with a proven gunsmith is a must. The initial investment is high. Getting the right components in the best configuration is the goal. Yet, this is no guarantee that you'll be sitting at the top of the ranks. The barrel may not be right - the "right" lot of ammunition has to be found, etc.
And then, it all boils down to the person pulling the trigger. You can have the best rifle ever built, with the perfect lot of ammo and still not hit the winner's circle.
But that's the challenge of this wonderful sport. When everything works together it's a thrill.
Best,
Michael