Because you need Office even if you only use it one hour every month?

Look at the gym industry, which makes its money off people not using their memberships.

Software isn’t gym though, it can be “used” by just keeping it in the background for multiple sessions in a tab somewhere, especially if the software does some monitoring so it’s always active.

Whether this makes sense though depends on the software.

For me personally I would not terribly mind to pay Photoshop by the minute because I would need it an hour per year. That’s extra cash that Adobe now just does not see. Most professionals though would still keep paying monthly because they use it a lot.

How much are you willing to pay for your hour of usage per year? $1? $10? $100?

Is the amount of work it would take to implement a metered rate at a rate users would pay worth it to capture the people who don't use it as a regular part of their job?