Headcanon: reasons for the Iacon 5000
- Sentinel Prime likes watching the race and he is not going to deny himself anything he wants, the whole point of taking over was to live like a king
2. Bread and circuses: it keeps the masses happy and compliant to have something to distract them. And it is working, even the miners idolize Sentinel.
3. It's dangerous, which means some Transformers will die, which means less energon will be needed for the city. (But tbh I think Sentinel would want it to be deadly regardless, for the thrill.)