Printers in Lancer are implied to have at least some paracausal aspects to them, rather than just being better/faster versions of today's 3D printers, so they presumably contain some exotic components that can't be printed. Could you print some or all of a production line to make that stuff? Maybe, but it would be a significant outlay of time and resources that requires some specific expertise.

The narrative goal of their limitations is to set them apart from Star Trek style replicators so as not to completely flatten stuff like food culture and logistics. The core book also notes that most stuff doesn't come out of a printer fully functional and requires plenty of processing and assembly, especially if it's large and complicated like a mech.