I think this happens because people don't treat the testing code as "production code" but something else. You can have senior engineers spending days on building the perfect architecture/design, but when it comes to testing, they behave like a junior and just writes whatever comes to mind first, and never refactor things like they would "production code", so it grows and grows and grows.
If people could spend some brain-power on how to structure things and what to test, you'd see the cost of the overall complexity go way down.
