Because debt is grossly under-priced and money has to go somewhere.
The artificially very low-to-negative interest rates have side effects all over the place. A big one is a grossly inflated equities market. People need places to park money when they're saving it. Retirement accounts, pension funds, financial cushions that people and corporations are *supposed* to have, etc. This adds up to many trillions of dollars. Normally, a good mix of this would go into CDs on the retail end and high-grade bonds on the institutional scale - they tend to be relatively safe while offering an acceptable return. With debt under-priced, bonds are a much-less attractive investment. The safer bonds offer poor returns, and if you're going to riskier places why not play the equities market?
Equity overvaluation then becomes a crazy cycle - cheap debt means its easier to fund sketchier startups, companies start making more money trading their own stock than they do in their normal business operations, etc. CEO salaries skyrocket because one of their biggest jobs is to sell their company's stock.
