Why do most of the companies not want to implement full HR functions, policies and procedures ?

It depends on the organizational capacity and size in terms of full HR functions and In terms of HR policies and procedures, from my opinion I'd highly recommend that the HR policies and procedures to be laid down to be ready as i believe there is a mission and vision for the organization itself for both business growth as well as manpower growth 

It depends on the organization capacity means structue, nature, employees and space. If organization has more than300 employee, they can run full HR.