I think a big part is just technical debt acquired through the years, when you are just starting you hack some version of the site that works because you need t

I think a big part is just technical debt acquired through the years, when you are just starting you hack some version of the site that works because you need to grow fast, but that's not scalable, so a part of becoming a big company is to "refactor" or "re-architect" your app to either more modern designs or in some cases into a completely different app (On the inside)