trading scenario - FI dco not generated when doing PGI

Business Scenario,

SD - MM

Sales inquiry -( when saved , PR is generated in background)

RFQ to vendors w.r.t PR

price maintained in RFQ

Sales Quotation to Customer and accepted

Sales Order created

PO with RFQ created to vendors( account assigned M )

GR - ( stock taken in for sales order stock, mat doc and fi doc generated.)

IV done

Sales delivery and PGI ( sales order stock is reduced and mat doc only generated and NO FI DOC GENERATED)

Billing is done ( FI doc posting happens )

please understand and help me to understand why FI DOC is not genrated while doing PGI