Apart from anything else, commoditizing your complement is good business strategy.
Customers are not looking g For silo systems. They're looking for things that play nice with sll the systems they currently have.
Mind you, you absolutely Can get into a negative loop - needy customer with unrealistic expectations who over pressures the sales person, who then over pressures the pm, who then over pressures the team and you end up with unfeasible expectations and conittments. But sanity needs to prevail everywhere, it cannot be focused on the "good kind of salesperson"
(I will never ever ever be a sales person; and I share most people's frustrations, but I've also seen what the good ones do for the customer and teams so am slowly starting to understand both the value and necessity :-/)
Equally though, the old phrase "we all work for sales" always rings as true now as it ever did. The best engineer with the best product will make diddly squat if they can't sell the product. The best salesperson could sell rocks off the roadside if they're good enough - the product is often irrelevant. Personally, I wish more engineers went into sales because the best salespeople have deep product knowledge and are way more empathetic because we've gone through trials and tribulations.
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.