Decision of Outsourcing stems from the fact that generally those tasks are outsourced which are either Non-Core or just one-time jobs
Accounting firms can outsource their website development or their social media management but cannot afford to outsource Accounting or Tax Preparation of their clients. These are their core operations that require approach, protocols and quality controls of their own firms while performing them.
For accounting firms, Outsourcing core business operations might result in Poor quality of work product and client deliverables.
Accounting firms often misunderstand offshore staffing with outsourcing. Offshore Staffing is hiring staff that works remotely (i.e. our office in India) outsourcing means hiring a vendors does portion of jobs