I'd guess because:

* It takes months for an engineer to be properly ramped up on a new codebase and even longer for business domain expertise. Until then they might actually be negatively productive due to onboarding questions other devs must answer.

* Enforcing any coding standards is difficult so you get a lot of technical debt over time.

edit: Also, a good engineer on a well functioning team will push back on bad or inefficient feature requirements. If you're just getting paid by feature and feel no ownership of the product then you have little incentive to do that.