Good practices and clean code paradox

I'm wondering, that since all good practices, clean code principles, patterns, etc. are to simplify things, why there's paradox that they are so easily overlooked or ignored? And we often ends up in big ball of mud?

I've seen more spaghetti code than clean code during my career.

Is it because these principles are difficult to apply in order to make things simpler? Or abstraction thinking is a rare gift, and devs are missing it? Or is there other reason?

I don't buy argument that messy code is quicker to write and deliver, as no later than 2-4 weeks of a project it's actually opposite – good code starts to pay off.

So, why?