Why don't the Government employ a set of Teachers, who are the Creme de la creme in their field, and task them with writing all lesson content, for all pupils, for the whole of their primary education? These could be all cross checked to make sure they are perfect. Then upload them to a central library, that all teachers can access and download. That way we would know that we are teaching the right thing in the right way, and that every school is teaching the same verified content, AND teachers would not have to write any content, just familiarise themselves with the lesson before teaching, thus meaning they can actually have a life!
Because each class is different. What works for one class does not work for another, so while the content is the same, you will have to adapt activities to the nth degree to account for the level of energy, understanding and engagement of the students. One class loves football while another much prefers basketball.
Because each classroom is different. I teach in a classroom where I have a whiteboard which is at best a screen (which I can write on with my pen), while other colleagues in the building have a projector on a white screen they musn't write on, ever, while others, again, have a fully finctioning interactive whiteboard that does all sorts of funky stuff I can only dream of employing. We have a computer suite where two students have to share a cumputer running windows 2016, another where students are using MACbooks and another where each kid has access to office 365 fully accessible.
Because each school is different. Not every school buys into the same resources, not every school can (hell, they can't even patch of their roofs). We'd have to build standard schools with standard resources from scratch. I have worked in 10 different schools in my practical subject and have had to adjust practical lessons almost every time because equipment wasn't available that I'd used before, or some was broken, or they used something else instead. Far too expensive to standardise all that and by the time you have, the curriculum will have moved on again.
Because every area is different. You absolutely cannot teach the same content in the same way in an inncer city school as you can on the countryside. Priorities are completely different, as are life experiences. I can use the example of York dungeon in York, but talking about it in Plymouth would be dry and unrelatable. Country kids may be far more invested in learning about farming while those in inner cities would look at you like a piece of toast if you tried approaching it the same way.
So you have core content (a National Curriculum, say) and allow schools to adapt as needed.