Why are MSA, SPC, FMEA, APQP and PPAP only core tools as per IATF 16949 and not other tools?
IATF 16949 is an extremely thorough programme that pretty much covers everything. The following tools are closely connected with one another and with many additional tools: MSA, SPC, FMEA, PPAP, and APQP.
You can take care of metrics if you cover MSA. SPC focuses on fundamental features of variation and has a close relationship to capability. DOE is mostly covered by Design FMEA and its pre-work in DFSS (Design for Six Sigma). The majority of Define-Measure-Analyze-Improve (from the DMAIC technique) that is left out of MSA and SPC is covered by Process FMEA with its pre-work and post-work.
When properly executed, the grand strategy of FMEA comprises all root cause analysis, solution creation, solution implementation, and controls.
In addition to covering Control Plans, PPAP also incorporates them with other tools and feedback loops.