In some of the companies I've worked at, design was the last priority and at some point, making the effort to switch to a proper design system and do the front-end migration seemed too daunting. So it was never prioritized, until the exit.
I doubt there's a formal answer to this, but to me it's like 'branding': won't give immediate results but compounds over time.
And if you're a company who's never done branding, catching up after 3y is possible but super hard.
At the end of the day, I think it depends on the founders' sensibilities, because with no measurable ROI there are all the reasons in the world to deprioritize that!
I think it's never too late to implement a design system, interesting fact is the more you're waiting the more work you'll have to do 'cause you'll have to deal with more migration. At least one way to tackle it is to start with the foundation like a visual language systemised.
In some of the companies I've worked at, design was the last priority and at some point, making the effort to switch to a proper design system and do the front-end migration seemed too daunting. So it was never prioritized, until the exit.