Why has credit quality improved since the summer of 2020?

Since the summer of 2020, the trend in default rates and in upgrade/downgrade ratios shows an improvement in credit quality in the United States and the euro zone. What can it be attributed to? The upturn in profitability, thanks to state aid, the slowdown in wages and the fall in interest rates; The fact that corporate debt net of cash holdings has not increased; The reduction in borrowing requirements due to the fall in investment. The trend in corporate credit quality is therefore consistent with corporate behaviour.