Sometimes they are. I've seen several applications that were successful for many years but eventually had to be rewritten because some vital dependency was no longer viable.

In web development we had an early generation of web apps that used plugins like Flash or Java to do things. Fast forward five years and those plugins have been brutally killed off by the browser developers. However other web technologies have become viable alternatives for some of those things. That's a big rewrite.

Some programming languages have had big jumps that weren't entirely compatible. Python 2 to Python 3 is an obvious example that took years but eventually resulted in not only Python 2 no longer being supported but some libraries never being updated to support Python 3 and others being created to provide similar functionality. In this case many of the direct code changes could be automated but you can't automate swapping out each obsolete library for a replacement with a similar purpose but a different API. And maybe you wouldn't want to because in the 5 or 10 years since you built the last version new ideas have come along and you're better off adopting them instead since you have to make a big change anyway.

Granted, that concept can and should be introduced into old codebases. Last year my team successfully warded off the Sirens of Rewrite by just doing the hard work of extracting all of the dead framework calls and then NOT adding them back in drag-and-drop style, but properly exposing them through interfaces that don’t require everything to know everything about the particular replacement framework we used.

"If" is the operative term here. "If" it is well designed, then there is usually not alot of reason to scrap it in the first place.