out of curiosity, what is the issue with sandbox games?
Obscure and repetitive gameplay (grindy). Dev tends to add grind as time filler to extend gameplay (I can assure we wont blame dev for short gameplay knowing that next release in vicinity, but rest assured everyone will curse out the game once visit same location dozen times just to see subtle change in dialog). All those triggers based on combination of day time and week days, some devs go further by counting exact hours as a result convoluted quest line (devs do sandbox games to run quests, right?) Personally, I can't name any successful sandbox game that would not be screwed out by accumulated complexity in year or two of its lifetime with fading interest of players. Also, focusing on quest implementation, leaves less time for quality story development, as a result most of sandbox games suffer from lifeless/bogus situations, i.e. find candom, lube to fuck someone.
Also, 99.9% of adult games are delivered in increments (releases). Therefore delivering game with linear gameplay is more practical ether from developer or player perspective.
Players only appreciate content (quality wise), not a gameplay it is delivered trough. I am not saying gameplay is irrelevant, but it only adds if implemented without sacrifice to content, single man shop will soon or later push boundaries here.