Because recruitment from TSMC to China is not an ordinary labor market. TSMC vs SMIC has huge geopolitical implications for Taiwan, China, and the world at large even. Most companies don't have "CCP's intelligence services are going to spend millions to entice, bribe, or coerce our key engineers to jump ship" as part of their recruiting concerns.

As for point 2, TSMC's financials are public. They spend ~30 million a year on payroll. On the other hand they expect to invest a total of 100 billion into their next generation fab over the next 3 years. The idea that penny pinching salary is what enables their capital spend is hilariously out of touch with the scale of the relative expenses.