I'd challenge the idea that customer support is _expensive_. Generic customer support requires no specialized expertise: you need knowledge your company's business processes, tooling related to third parties they worked with (redirecting a courier's shipment via the courier or whatever), and general customer service skills, and that's about it.
_Specialized_ customer support, e.g. support for complicated software, is expensive, because it requires technical domain knowledge equivalent to or above many engineering employees, and employees that can perform it effectively could probably move to engineering and increase their salaries twofold because compensation isn't tied to expertise, it's tied to what the market will pay, and you can more easily get away with shit support than shit engineering, but ignore that--it's not the majority of customer support.
Customer support _is_, in the simplest view, a cost center. Customers contact support when something has gone wrong that they can't resolve otherwise. If nothing goes wrong, you don't need it. If you, as support management, can reduce the time it takes to resolve things that go wrong (by any means) and reduce the number of staff needed to handle cases, you win: your department doesn't directly generate revenue, so your goal is to reduce expenses.
Tying lost revenue to customers lost after poor support experiences is difficult and noisy: there are plenty of customers who have poor experiences because they're incompetent and wouldn't succeed with any amount of support, and because of this are more likely to request support, and the high-level categorization used to describe issues often obscures what actually went wrong (a generic "Account > Creation" category isn't going to capture issues like "users can't create an account if their address includes non-ASCII characters", but execs don't see anything more than the category).
These issues (and others) combine to promote an environment where support is more a buffer to getting support with limited ability to fix issues, and a perverse incentive to _not_ fix issues since metrics are focused towards reducing time to resolution (easy to measure) versus quality of resolution (hard to measure!). Poor support is furthermore easy to ignore because you can either often focus more on new business first and foremost (hello, every startup ever!) or rest comfortably on a monopoly position where customers can't drop you even if you provide terrible support (hello, the US ISP market!).
Cost is not strictly an disincentive to providing sufficient, capable, and effective support, but it's difficult to recognize the value it provides and there are more immediate cost savings from reducing support cost than long-term revenue from customer retention by providing good support.
This compounds into even poorer service in more specialized environments because talented people will leave, because working in a cost center sucks. The flipside of support is essentially sales engineering, AKA support while trying to court a customer. The technical skills needed aren't any different, but one brings in revenue in an easily quantifiable way, so it gets more political clout and more organizational investment.