Because these devices are usually being supported by teams of only a few people. Eventually someone decides that a new project needs all hands on deck to meet deadlines and people get reallocated internally from the "least important" projects.

The fixes are simple, but they all share one problem: they cost money. Hardware orgs are almost universally run by the most penny-wise pound foolish people imaginable, so that problem is completely insurmountable to them.

There's other issues too (e.g. upstream vendors don't like upgrading and charge absurd amounts of money for it), but the main problem is simply a lack of institutional priority to do the work.