More addictive than crack or opioids, corporate debt is the sand-castle town's equivalent of water: it holds the 'marvels of castles' together, util it no longer does...
Firstly, as @Lisaabramowicz correctly summarises: "American companies look cash-rich on paper, but average leverage ratios don't tell the story. 5% of S&P 500 companies hold more than half the overall cash; the other 95% of corporations have cash-to-debt levels that are the lowest in data going back to 2004". Which is the happy outrun of the Fed and rest of the CBs' exercises in Quantitive Hosing of the economies with cheap credit over the recent years. So much 'excessive' it hurts: a 1 percentage point climb in corporate debt yields, over the medium term (3-5 years) will shave off almost USD40 billion in annual EBITDA, although tax shields on that debt are likely to siphon off some of this pain to the Federal deficits.
Secondly, this pile up of corporate debt has come with little 'balancesheet rebuilding' or 'resilience to shocks' capacity. Much of the debt uptake in recent years has been squandered by corporates on dividend finance and stock repurchases, superficially boosting the book value and the market value of the companies involved, without improving their future cash flows. And, to add to that pain, without improving future growth prospects.