The plastic should be moulded and then metal easily cut from sheets because it needs to be cheap and mass produced.
The cost of the plastic being mass produced would be a high cost but because lots are being made they can sell lots are a cheap price.
You could substitute the process with 3D printing or CNC but the cost for production would be a lot higher and more inefficient.
While CNC would be that it could be automated, it would cost a lot more for maintenance and production.
The fastenings have been brought as standard components and fitted to the component. This is the cheapest method instead of making your own.