This boils down to simple economics. Most softwares built by startups run over the cloud (which incur recurring costs to the startup) and most softwares are not

This boils down to simple economics. Most softwares built by startups run over the cloud (which incur recurring costs to the startup) and most softwares are not just a one-time delivered unit like a physical object. They need patches and continuous improvements, which means it needs a supply of human intellect (which is again a recurring cost in terms of salaries) The challenge is how would you reconcile this underlying costs with a one-time-payment? It's a tougher problem than it seems, if yoy include other variables in the equation like inflation, volatile nature of softwares , etc. Nonetheless, this has to be solved. I strongly believe a simple pricing model that makes instant sense - is an important part of the UX. And a one-time payment like buying a physical product is the simplest of all.