in sales invoice we can set default the due date by choosing ‘by’ or ‘net’. Its great, and why you not add the same ‘set default due date’ to purchase invoice? That’s gonna help me 
The seems undesirable to me, because different suppliers have different terms. The risk of accidentally leaving the setting at the default due date seems high, and could lead to overdue payments, with adverse implications.
My view is that being required to consciously enter a due date based on the individual supplier’s invoice is, in the long term, going to produce fewer problems.
When you receive an invoice from supplier, do they explicitly show due date or do they only mention payment terms such as “Net 30”?
In my experience, business suppliers (of materials, services, etc.) typically state something like Net 30, often because they are using pre-printed invoice forms. More general suppliers, such as utilities, insurance companies, and the like often print a “payment due by” date.