Why's Federal Reserve buying individual corporate bonds, when corporations have been selling them successfully?
"...Its March 23 initial announcement is largely considered a watershed
  moment for the financial markets, reeling from the coronavirus threat
  spread..."


As the quoted statement implies, 
people are spurred into buying corporate bonds in the current environment precisely because of the Fed's previous announcement that it would purchase such bonds. The market has been---more precisely, believes it has been--- front running the Fed, which achieves intended effect of the policy. As shown by the examples listed, corporates have been able to tap into the bond market for financing. The Fed now has to demonstrate it can/will implement its announced program to signal its credibility, and thereby maintain the effectiveness of its current and future policies.