It's a mix of piece coat optimization and a lot of creep in what used to be a pretty lightweight process throwing it into the ditch.

The things that run software in cars largely fall into one of two camps: MCUs and SOCs. Think Arduinos and Raspberry PIs. Background programming, with an active and inactive partition, is absolutely possible on a SOC. They're even file based, so you can do all kinds of clever things. Cars tend to not have many SOCs, so it's not a monumental task to pitch having them each coat a little bit more for extra storage/processing. The biggest hurdles here are automotive grade and the very long development cycles. These both mean that the hardware is 3+ years old when it launches.

MCUs tend to have monolithic software builds (think literally everything gets compiled into a single .exe). There are a million billion of these things in a typical vehicle from most automotive OEMs. It's... very hard to make them all have more capacity because you would take that cost and multiply it by 40 or so to get all the MCUs on a vehicle 'upgraded' for extra capacity.

If this all sounds a little crazy, it is. From two angles. First: do we really need as much software control in cars as we do? Marketing departments seem to think so. Second: the reason why there are so many small compute units in a car is the slow migration from mechanically controlled components to electrically controlled on. Back in the 80s the majory of automatic transmissions shifted based on a very complex mechanical system (look up a transmission valve body if you're curious). Moving that to electronic control meant adding a computer to control that functional. Now take this and multiply it and you'll kind of see the wreck in motion. Most OEMs are moving toward more centralized compute (fewer, larger, and smarter control units), but new electrical architectures take a lot of time/effort so it's slow going.