I have started companies 15+ years ago that I sold which still use (a lot of) the same code. You think (I thought) that would never happen but I think this idea

I have started companies 15+ years ago that I sold which still use (a lot of) the same code. You think (I thought) that would never happen but I think this idea that every company rewrites everything is not all that common. Banks don't, but the small startups I work with don't either. Frontends get redone, there is refactoring and library updates but most (unless trivial tiny systems) just stays the same. You need to think that they run a business and that business is not software development usually. So if there is not a pressing reason to replace things, why would they allocate money for that?