If uber paid you $15123 but you:
Just bought a new bike bc your other was stolen
You paid $1200 for insurance
You bought a helmet and cold weather clothes etc etc.
Those things reduce your taxable income.
Other countries presumably rely on other fraud signals. They might have more visibility into your day-to-day financial transactions, or there might be more of a culture of leaving an anonymous tip if you suspect your neighbor isn't paying a fair share.
They have simplified it nicely, though: if you work from home you can claim a per-hour deduction so you don't have to do the math of wear-and-tear, electricity, internet etc. I think it was $0.6 per hour?
Thankfully(\s), they are simplifying it even further next year and removing whole thing. Now you only get to deduct money if you actually rent an office...