I do not understand the problem. The ‘kit’ system is an industry proven model that fits national standards of safety; reliability; ease of manufacture (therefore relatively cheap) and most importantly is certfifed.
Build your own panel by all means and then go to the trouble and cost of certification: fault level testing etc., and in the USA you will also come up against the ‘Arc Flash’ requirement. In the UK the DNO engineer will need proof that your panel complies with all relevant safety and operational requirements.
Adding functional units is something that is considered at the design stage and should not present any difficulty to a competent panel builder – if the panel board is full add another board (and this time allow for future expansion ).
Using the manufacturers equipment is surely a good thing; it provides interchangability; a lower spares stock (inventory), traceability of performance data.
The cost and packaging are surely not the issue – relaibilty is.