It's easier verifying that scenarios are legit because we only need to be familiar with how the tune in the scenario performs, not how every single tune in the stock game performs. This would make verifying time trials into a full-time job. So, if we were tasked with making sure half of the runs are actually using a stock tune, we'd need to run 30 minutes of tests every time we verified a run, just so we know if it is or isn't a stock tune.
What we don't have memorized is how every single of the hundreds of default tunes perform. When verifying trials now, we know about what each car looks like maxxed out, and what is further than that (so hacked or cheated). It would make verifying runs for time trials almost impossible, and it would require runs to be submitted in a very specific format.
