This is a product of the regulatory process. I am sure Cessna would love to sell new stuff, but it's just not worth it for them or their clients.
If I recall correctly, Cessna was found infinitely liable for every plane manufactured, meaning that each new plane off the assembly line increased their potential overall liability without liability over decades-old planes expiring and decreasing it.
This was a problem in the 90s. And it was fixed! Back when the government still fixed things. https://en.wikipedia.org/wiki/General_Aviation_Revitalizatio...