It's not really laziness. Storing as JSON solves or prevents a lot of problems you could run into with something bespoke and "optimally packed", you just have the tradeoff of needing more storage for it. Even then, the increased storage can be largely mitigated with compression. JSON compresses very well.

The problem is usually what they're storing, not how they're storing it. For example, The Witcher (first one) has ~20MB save files. These are mostly a bespoke packed binary format, but contain things like raw strings of descriptions in multiple localisations for items being carried, and complete descriptors of game quests. Things that should just be ID values that point to that data in the game files. It also leads with like... 13KB of zero-padding for some reason.

looking at you x3

and rimworld

I wouldn't say bad, but inefficient might be fair. Unoptimized I think is more representative.

Inefficient/unoptimized would be an accurate description. I think it's important to add, for bethsoft games specifically, that the save includes all changes to objects, even if the player themselves didn't interact with them(e.g. Physics interactions, explosions moving things, npcs bumping stuff around), and also includes all NPC changes. Master files(ESMs) get loaded, then the save loads the changes it has baked in to the databases. So, when you load up a save that has traveled the world and loaded a lot of things into save memory, the engine has to sit there and reconcile all the changes with the ESMs, which can add up quick if you're playing modded.