Why Salesman cannot deliver

Currently I am orchestrating all workflow of web development process, ie. helping sales team to sell and championing development team to deliver value to every our client. In my past experience, we always had 2 separate roles: sales, product owner (PO) and never mixed or combined them together. However, summing up several most recent web projects I double confirmed the conclusion, that sales team should sell and product owner with development team should do their work and take over the communication with the client once the client signs the contract. PO should manage project scope, timeline, deliver constantly, get directly feedback from the client and adjust thing on the go. I have some examples, when salesmen screwed to get the message to the development team and were not able to get clear requirements or to say "No" to the client, as a result, the client got into the situation when engineers don't know what they did wrong, salesmen say that engineers have not created what client expects and multiple fires are burning.
We are adjusting our workflow, we are passing the rod from Sales to PO in the early phases of the project. Sales teammates and Product Owner with development team have black belts in different areas.