The case for Apple is sort of obvious, the Mac Pro has always been slow to rev and has always had slow sales. Apple has also been transitioning to their ARM CPUs so they're not going to double down on x86 on a single line of machines.
I'd imagine other manufacturers are in a similar boat. They put in orders for Skylake Xeons for machines that sell slowly. Pandemic sales went into laptops for home offices rather than $5k desktop workstations. Desktop workstations have never flown off the shelves but are flying off more slowly than normal.
Ryzen and i9 laptops and a beefy VPS have been good enough for many WFHers so no one is buying desktop workstations. Manufacturers aren't going to jump to new Xeon parts if they're sitting on old stock.