I think it comes down to scale and cost.
I've spent a bit too many hours in a call center, both as an employee, and a few years later developing the software that ran one.
In both instances, there were people there who really knew what they were talking about, and a lot more who read from a script. I was a script-reader my first month or so, before moving up the chain, and I was shocked by how often the script worked. Something like 3/4 of my calls were resolved with a one-line fix in a knowledge base I could search.
As I moved up, so did my pay, and I cost the call center more per hour (nearly double). So it made sense for them to a.) not have very many people like me and b.) to make sure that my time was spent only on the problems that couldn't be solved by following the script.
In fact, hell hath no fury as the per lower-level call center person who forwarded through a call to me that could have obviously been resolved if they'd just followed the script! I'd berate them in notes for not reading the numerous knowledge base articles I'd carefully prepared, or for not bothering to check a few simple things clearly outlined in their training before even engaging the script, or searching the knowledge base. They'd just wasted everyone's time. The customer who sat on hold waiting for me, the manager who had to approve the escalation, mine because I'd spent thirty minutes assuming they'd done their job when they hadn't (and double-checking just made the customer furious if, in fact, they had.)
You might think that it makes some fiscal sense for companies to ONLY hire people like me, and eat the cost as a marketing thing, or accept that by offering great support, they'll get better customers. But you literally could not find enough support people like me to handle all the calls if you tried.
So you hire relatively dumb, but friendly and resilient people, because that's what you can find. They're cheap and readily available. If one of them shows promise, you promote them. You try to find experts that are desperate for the job, but they're few and far between and generally come with baggage.
You work with what you can get, and optimize around it.