Having worked with the "big scary companies", I can say they are 100% to blame. It doesn't start with the developers but rather the budget. Unless folks at the top are tech savvy and/or have an engineering background, they typically only budget for new features and either under-budget or don't budget for maintenance and tech debt removal. And when they do budget for maintenance, it's handled almost exclusively by "maintenance teams" that are offshore and cheaper.
So you have a feature team that works on a feature for 6 months, does a 1 hour "KT Session" with the offshore maintenance team and hands them the code. The offshore team has some information on the feature but not enough to really manage existing tech debt, just to keep the lights on. And on top of this they know they are the lowest totem on the pole and don't want to get fired so they don't go out of their way to try and fix any existing code or optimize it, again just enough to keep the thing working.
Then this cycle repeats 100-1000x within an org and pretty soon you have a scenario where the frontend has 2M lines of code when it really should be 250k max. A new feature team might come on with the brightest engineers and the best of intentions, but now they have to work within the box that was setup for them. Say they have a number of elements that don't line up with their feature mockups. The mockups might be incorrect, there might have been an upgrade to the UI kit, or the existing UI kit might need refactoring. Problem is none of that is budgeted for so the team is told to just copy the components and modify them for their own use. And of course on handoff to maintenance team, the new team does not want to mess with the existing feature work so they leave it as is. Management is non-technical so they don't know the difference, and you end up with 50+ components all called "Button" in your codebase from years and years of teams constantly copy/pasting to accommodate their new feature.