On the surface it seems stupid to think that an un-refuted argument should be considered true. But arguments are evaluated that way in policy debates for a very specific reason -- the purpose of debate is not to discover the truth, it is to win an argument. That's the whole point of the activity. If you don't like it, you don't have to play.
That's actually the reason I quit during my sophomore year, but it doesn't make debate a worthless activity. Nothing taught me to work harder or think faster.
Debate is like working out. It's not real, so certain considerations go out the window. Under your logic, running on a treadmill is a bad idea because you're not actually going anywhere. But the point of debate isn't to discover the truth, it's to sharpen your understanding and wit.
Where debate goes wrong is when you stay in it too long. After a certain point the skills become harder and harder to transfer to other parts of your life. And a lot of the people in it, as you might be able to imagine, are pretty arrogant know-it-alls.
After I left debate, I had to re-socialize myself back into the world of normal people who don't argue with each other all day for fun. It sounds like I'm exaggerating, but all my friends were debaters and all my time was spent researching, debating, or arguing with them about literally everything. That's just how debaters tend to spend their free time.