TL;DR: It's a well-known fact that most people have games in their Steam library that they've never played – and possibly never will. It's a subject that one analyst has examined in detail, and essentially concluded that Steam is filled with hobbyists: people more interested in buying items to collect rather than consume. Hoarders, basically.
"The amazing thing about Steam and its player base is that they buy games they aren't going to play," writes industry analyst Chris Zukowski. "More than likely the person buying your game is not going to play it," he adds, addressing developers.
Zukowski compares Steam users to other hobbyists with "piles of shame," such as LEGO builders, Warhammer collectors, and knitters, who amass unopened sets, unpainted models, and unused yarn, respectively.
A few years ago, Simon Carless wrote that just over half the games in the median Steam player's library sat unplayed – a statistic that has been put forward since at least 2014. For many people, especially those who like to splurge during sales events, that figure is likely even higher.
Zukowski notes how Netflix once complained that it was trying to sell to people who had an endless number of entertainment options at their fingertips but not enough time in a day to play or watch it all. The analyst says that Valve solved this issue by adding an infinite number of hours to a gamer's day: a theoretical future when they might one day spend ages playing these titles – even though they probably won't.
"The reason game developers are willing to give Valve 30% of their revenue is because the Steam marketplace is packed with super gamers who throw money at games they have no intention of playing," Zukowski said.
A good piece of advice Zukowski offers is for developers to lean into a very clear genre for their games. This helps fans of said genres make impulse buys during sales as their rationale will be, "I like this sort of game, so will definitely play it some time in the future."
The article concludes that developers should not be discouraged by the thought that people who buy their game might never play it, as such a scenario is totally normal for Steam. He added that it's the reason why devs give Valve its 30% cut of sales: "you get access to a bunch of drunken sailors who spend money irresponsibly."
Zukowski makes plenty of good points, though most of them are ones people already knew – and some gamers could object to being called drunken sailors. He himself admits that two-thirds of his Steam library remains unplayed.