The salesperson has a vested interest in getting the customer everything they need to win business so they get their commission. A good one is an utter pain in the ass internally because servicing them and the customer is quite often at the expense of everything else. 1:1 sales tactics don't scale unless the customer is huge. It's a classic 80:20 rule - 20% of customers normally drive 80% of revenue, so it's only sensible to focus on certain customers and ignore others - but obviously that only makes sense in a vacuum where businesses don't rise and fall, and suppliers alongside them, so you need to toe the value threshold when it comes to exposing customers to engineering - which is where a good salesperson is gold.
Product designers, developers, executives, and support staff - all of them have better things to do than service a single customer most of the time. Building features, marketing, hiring staff, documentation, validation, and making new products. All these things are 1:N activities. If they're sucked up talking to one customer, they're not doing 1:N. It quite often feels like the salesperson works for the customer, not us, because it's clear to everyone that the demands are hurting the product in the long run by sapping everyone's time doing servicing when the opportunity:cost ratio isn't there.
To your point, it feels great when a customer follows the expected flow. Planning/marketing/engineering/executives can be brought in as and when needed, rather than when they're demanded of. Almost all customers demand the time and attention of everyone but the sales team, but I've often found that the ones who provide all the info and jump through all the hoops get what they want much faster.