Metering software by the second seems worse for both the developer (whose revenue becomes less predictable than e.g. monthly billing) and the typical customer (who now needs to think about whether the particular document they want to write warrants paying for Office, or whether it's fine to just do it in GDocs or LibreOffice).
It benefits someone who just uses Excel for a short time to do their budgeting each month (because it saves them money), but it also decreases the value of that customer so much that Microsoft probably doesn't care that they get (wild guess) 5x as many of them.
It would make sense in fringe cases, but not for mainstream use.
Purchasing access to exclusive content with pre-paid minutes is not a new thing after all, but most people want either buy-once-pay-once or "manageable monthly/annual rate", not a highly variable and potentially very spendy financial timebomb ticking away on their computers.
It benefits someone who just uses Excel for a short time to do their budgeting each month (because it saves them money), but it also decreases the value of that customer so much that Microsoft probably doesn't care that they get (wild guess) 5x as many of them.