EVs are at their heart really simple machines. You have a battery, a battery controller, some kind of thermal management for the battery (can be passive but active is better), and an electric motor that isn’t much more complex than the one in your vacuum cleaner. All these parts are readily available, so you can easily convert an ICE car to an EV with parts you can buy yourself. For some popular cars there are even kits available these days, and conversions of old-timers are really taking off. If there’s a kit with clear instructions a conversion can sometimes even be done in a single day.
An ICE car is literally orders of magnitude more complex. There are far more parts, far more computers to control all those parts (in a modern car that is), far more cabling to connect all these computers and far more opportunities for things to go wrong. ICE cars even spontaneously catch fire more often because of all the cabling and complexity.
You’re probably thinking of a few specific brands, like Tesla, that are very hostile towards self-servicing, and in those specific cases you are right. But most EVs from established car manufacturers are just as proprietary or non-proprietary as their ICE brethren, just with less complexity and less parts. And of course older cars that don’t have any computers in them are immune to this specific kind of complexity as well.
Millions of Steam Decks and their ilk have been sold, and run games significantly better on SteamOS than their Windows counterparts, to the point that Microsoft is reportedly cancelling their own gaming handheld plans. Not a massive challenger to the Switch 2 for most ordinary people, but things are definitely changing.