Steam doesn't allow pre-loads on games already in early access.
I feel like that could be easily overcome by releasing a new game rather than updating EA, and giving anyone with the EA prior to the release date a free download.
It could be done, but Steam would likely charge a significant additional fee for administrating the second version of the game, storing it, distributing it, supporting it etc etc for what is realistically zero benefit to Larian as a business. You could argue Larian might outlay that finance for the customer experience, but honestly if they did no-one would be patting them on the back for it as hard as they are dumping on them for not. I suspect they already financed Steam to get staff in to work early on the 3rd to support an 8am launch over the original 10am, and that has gone virtually unacknowledged as even happening anywhere.
I also believe that where Steam do not offer preload they actually require other platforms not to, so that would cover GOG too.
