This is a good question! The most common reason I hear in SaaS is that the arbitrary time window creates a sense of urgency for the person who has signed up to use the tool. Whereas, if there's no "countdown clock" people are more likely to forget about the software and never activate.
A lot of companies do Freemium versions or Reverse Trials where usage counts down based on user activity instead of time for the reason you're describing. But it's way harder to get people to upgrade from Free than to get them to convert when a time-based trial ends, generally.
A lot of companies do Freemium versions or Reverse Trials where usage counts down based on user activity instead of time for the reason you're describing. But it's way harder to get people to upgrade from Free than to get them to convert when a time-based trial ends, generally.