For most large commercial use cases, there is typically negotiation and customization involved. Very rarely will a standard SaaS program fit right into an enterprise customers work stack just like that. There are many other requirements that won't be covered and on the flipside, there could be many other ways to structure a deal that works for both parties. "Contact us for a quote" helps companies to build relationships with customers and understand their problems instead of proposing a take it or leave it scenario.