You mean the 2009 OS that mainstream support ended on January 13, 2015? I am shocked that Obsidian isn't supporting Windows 7.
Yes, the 2009 one that nearly 40% of the desktop market still uses and has no desire to upgrade. Now that Obsidian is Microsoft they won't support their older OS's. Keep sucking up I'm sure they're just about to make you a mod.
Around 30% of Steam gamers use Win 7. Less for gamers in the West, the target market. Less among FPS gamers. Obsidian, Private Division, and Microsoft will have statistics on the market, they won't leave 40% on the table. Microsoft don't decide on OS support they're not publishing. Extended support ends 2020, desire to upgrade is incoming.
As much as it might shatter your entire sense of reality to absorb this fact, not everyone uses Steam as a platform so it isn't a proper metric. Current estimates are 40% regardless of what Valve says.
Microsoft owns both State of Decay 2 and Sea of Thieves also, they didn't release either for Windows 7 or 8. Microsoft's studios only support the current OS because that's how they roll.
Have you ever heard of statistical sampling? Steam platform offers enough of a sample to provide a view into the whole gaming community. So, %25.
And I'd wager a good sum that most of these devices are old systems, unable to run the game in the first place. Since aside from the mad haxxors with some odd ideas about freedom and some just simply stubborn folk, everyone with the technical knowhow already upgraded to Win10.
And finally, they obviously ran some numbers, decided Win7 wouldn't have good return on investment, so they didn't bother. What we think does not matter here. Only the bottom line matters.