There are two main reasons why operators don't invoice:
(1) evading state taxes in order to cover up the real turnover.
(2) In case of disputes with consumers about the quality of goods sold or services provided, in order to deny and pass the buck.
Legal basis: Article 20 of the Measures for the Administration of Invoices in People's Republic of China (PRC). Units and individuals that sell goods, provide services and engage in other business activities collect money from outside, and the payee shall issue an invoice to the payer; Under special circumstances, the payer will issue an invoice to the payee.