I see an absolutely shocking number of managers promoted from the IC ranks, who not only have no preparation for management, but no experience at any other company.
You are describing the best kind of manager for two reasons:
1. They understand what their reports do, can mentor the less experienced ones, and are a competent peer to the more experienced ones, rather than an obstacle.
2. If they turn out to be bad managers, there is a low stakes, no hard feelings, path for them to go back to being an IC. There is a huge aversion to firing people, so bad managers who can't do anything else tend to stay around creating problems much longer than bad managers who can also contribute.
Your presentation of "experience" and "preparation" as the most important things for a manager is typical gatekeeping that we see from the bureaucratic class--parasites without any real skills.
There is no guaranteed way to create managers from scratch, business specialists don't understand the technical facts well enough to resolve the kinds of disputes that arise at the project manager level, and as you observe ICs are not always inclined to make other people's work their primary concern.
The alternative of professional managers who never did anything didn’t work out so well either, especially in engineering.
I've encountered both good and bad managers who were promoted from individual contributors. A key difference is whether they wanted to be in management, or whether they found themselves forced into management because there wasn't a good technical leadership ladder or a good opportunity to climb it.
That's inevitable given how quickly the ranks of software workers have grown in the past 20 years.
I wonder, what do you see as a desirable alternative?
In the (US) military, the sergeants run the army. NCOs are highly-trained, and have been the secret of managing battlefield chaos, for generations.
They don't do strategy, but they do tactics, like nobody's business, and are often highly valuable input into development of strategies. They are given tremendous agency, and are highly trained. The military does a great job of training and retaining highly-experienced, and highly-skilled NCOs.
First-line managers have a similar role, but they are treated like garbage by their superiors, and consider their position a "necessary evil," towards higher ranks. They don't like their jobs, and want to get out, as quickly as possible.
In unions, foremen are often quite happy with their roles, and don't really want to go beyond (they wouldn't mind more perks and pay, but they like their jobs).
Like bad tech career ladders, the manager career ladders are also pretty terrible.