Yes, because they make money selling the parts, and there are warranty requirements that are hard to fulfill if you don't have parts.

Often after a decade or so, companies will sell the designs to dedicated parts makers. For example, Volvo has Volvo Classic Parts, and they even have a reman program, and will even 3D print parts not available. Mercedes has Mercedes Classic Parts. Chrysler has MOPAR, etc.

Here you can browse parts for a 1968 Mercedes SEL: https://classicparts.mbusa.com/c-280sel-223

If you are a business, the costs of designing the part has already been paid, if you can sell the design and get some royalty payments, why wouldn't you turn those old plans into cash?

And of course there is a huge industry of Chinese clones and other suppliers that will provide replacement parts that are not genuine.

Be prepared to pay, though :)