I know lots of you have grown with it so that's just the way it has always been for you and you are used to it, but older gamers, why do you need a launcher?
I've started PC gaming in the mid to late 90s but only when visiting cousins and friends. Got my first PC in 2001. I have some original games but I'm like 99% pirate, especially for "newer stuff" (read: anything that came out in the last 20 years lol). Modus was always the same: run the installer, click the shortcut, play.
I created a Steam account sometime in the late 2010s, I remember I did because I saw they were giving Metro games for free and I wanted to play them, and I started collecting free games that looked cool, but it really really bothered me that I needed to open their store to install and play the games. Even if I made desktop shortcuts their program would run in the background, and usually complain if I was offline... I just found everything so useless... run software to run the software I want to run, why not skip the middleman? Also I have always been on shitty hardware and I didn't like that extra RAM consumption going on in the background.
Eventually I stopped using Steam, deleted my account, and went back to piracy, but with the loss of some trusted trackers and stuff, and me starting running banking and other important shit on the same PC, I decided to start buying games, and then I found GOG, and what a godsend store! When I buy the game I get the installer so I can do whatever I want with it, and I don't need any third party application to install or run them.
I see a lot of people saying they don't buy games from other stores because their launchers are shit... but what do you even need a launcher for? Not having a launcher is my requirement to buy a game lol
All of my systems are Linux, launching Windows games on Linux is not trivial, Steam takes away almost all of that complication. It also provides an excellent ten foot interface for me to use on my TV and buy/install/launch games from my couch without any hassle. Speaking of controller usage, Steam provides excellent support to remap controllers even if a game doesn't support it, and provide amazing features at that (for example multiple layers, gyroscopic mouse)
Games getting updated automatically is a great feature, I still remember having to download patches and applying them one by one after a fresh install. Similarly Steam also provides a workshop that allows you to install mods and keep them synced across different systems automatically.
Finally, the convenience of cloud saves for someone with multiple systems or who uninstalls a game and reinstalls it later is not easy to achieve without a launcher (I still have a saves folder backed up somewhere from before).
Besides all of that Achievement and other social features are important for some people. And for some games being able to easily play online with friends is amazing (if you're not old enough to know what GameSpy is you don't know what it was back then), although I don't play too many online games so this one is not that important for me, but when I need that feature it is very handy.
In short there are many reasons, but if you're playing old single-player games with mouse+keyboard on only one windows PC, then none of my reasons apply to you. Still I would argue that buying games on steam is easier than pirating them, so there's the convenience factor still (e.g. at a friend's house and they mention a game, open my phone, and in 5 min with a very intuitive flow I have the game downloading on my home PC so when I come back it's ready to play).