It's no surprise at all. Look at MMORPGs. The best moments of these games were pulling high level monsters to city, crashing an in-game funeral in WoW and the drama between players in Eve Online. Not farming monsters for 20 hours to get enough cloth. Unfortunately in many cases the unscripted emergent gameplay was often connected with "toxic" behaviour, as a result most games nowadays reduce conflict between players to regulated arenas, and provide a themepark experience outside.

I think one explanation is that the sandbox/simulation games take more time, leave players with 'more' memories, and as a result are more linked with particular periods in their lives.

Looking at my gaming past, a number of scripted experiences have actually left the most vivid memories, but the sandbox/simulation games are the ones I associate more with friendships, periods in my life, developmental stages, etc.

Nostalgic thoughts are about periods of time, and less about moments. A scripted game, I think, is usually too short to produce nostalgia. The few scripted games that do, like the Myst series, took an unusually long time (for me) to play through, for example.

20 years from now, how will our current gaming scene be defined in the compressed, excerpted view of retrospective history? Sure, there are scripted megahits such as "The Last of Us" and the "Call of Duty" series. But the standouts in terms of pure revenue and popularity metrics would seemingly be the Grand Theft Auto series (particularly V) and, for indies, Minecraft (pre-Microsoft).

Call of Duty isn't really more scripted than GTA, and arguably it's less; both have a scripted singleplayer campaign, but Call of Duty has always been a multiplayer game first and foremost. I have clocked many, many hours in CoD4 since it came out, but only six of those were actually scripted.

Kerbal Space Program without a doubt.

I think of the current scene dominated by a lower barrier of entry to market and the current rise of indie and "early access" games so maybe it won't be defined by games at all but by the dominance of Steam.

The 80's had an even lower barrier to entry, but now we tend to only remember the best that was on offer.

Not really, if you count "access to a computer" and "access to distribution channels" in those barriers.