This sucks for those looking to play the game on platforms/launchers it was advertised on before it was yanked at the last possible minute. There are ways to do exclusives, such as funding your own games or, at the least, making deals as early in advance as possible. This is not the way to do it, especially for a store looking to engender goodwill early on.
And before the tide of "What's the difference if you buy it on Steam or Epic Store? It's all on PC, lul" posts, I'll explain why I (and I assume many others) feel like it matters.
1) Loyalty to Steam and Valve for pulling PC gaming out of the niche downward spiral of the early 2000s. Like was said in other threads, while Epic jumped to Xbox to make exclusive console games, Valve revitalized the PC market, created a huge platform for indies, and helped fight piracy by making a storefront easier to use than just stealing the games (and offering them at sale prices I still lament and miss.)
It's okay if you don't care about this. Valve isn't "owed" anyone's loyalty. But it makes sense for some of us to feel it, at least a little bit.
2) Steam support for Linux, giving a bunch of people access to titles they'd otherwise be unable to play. This doesn't personally affect me, but I respect the hell out of it.
3) Steam VR being an open platform. Again, doesn't affect me, but I love this.
4) Supporting sale of Steam keys on other sites while not taking a cut. This helps keep prices down while still allowing me to own titles on my preferred platform/storefront.
5) Built-in community features. Steam is where the vast majority of my PC gaming friends are. Yes, I can add them on other platforms, but no one keeps all of their launchers open at all times. Many, myself obviously included, keep Steam open as their central hub. It also has built-in groups, although I use that more rarely.
6) Steam Controller support. While many aren't fond of it, the Steam Controller is my main PC controller. I love how easy it is to remap keys and adjust sensitivity in games that don't natively support it. Along those same lines,
7) Ability to remap buttons and adjust sensitivity on Xbox, Playstation and Steam controllers. This has been a godsend not only for allowing players to use any controller on modern games, but also for allowing them to play older titles that have iffy controller/joystick support with ease. It was a lifesaver when I played Prince of Persia: Sands of Time, for example.
8) Steam in-home streaming and Steam Link. It's so convenient for me to get the "comfy couch" experience while not having a second PC or a giant HDMI cable going through my house.
9) Steam forums. Yes, there is some toxicity, for sure. But it's also been responsible for surfacing fixes on a multitude of games I had issues with over the years, as well as informing me of titles that no longer work on modern platforms. Yes, I could have found this elsewhere, but having it built in makes it significantly more convenient for me to look into something (literally right clicking my game and clicking to the forum page.)
And those recommending Reddit as a less toxic alternative are kidding themselves. Yes, a good community can be created there with great moderation. But at that point you are trading having to moderate your Steam forum for moderating off-launcher, which is essentially the same for you but less convenient for players.
10) Auto updates. Yes, stores all now support this, but Steam helped introduce it. And, because it's my main platform (for reasons stated here and because I have, by far, the majority of my collection on it), it's the only one that keeps my games updated regularly. I can't count the number of times I went to play an EA game and had to wait for a giant patch before I could. I don't have that problem on Steam.
I'm aware this is a bit of a chicken-and-egg thing -- Steam is my main, so its games stay updated; it's my main in part because my games will stay updated -- but it's also the reality for me.
11) Regional pricing. Again, not something that affects me strongly -- I'm Canadian, so we have it but it's not a big swing one way or the other -- but it does affect a lot of people. When games cost triple in comparison to the average income than they do in the States, that really sucks.
And I honestly could go on, but I think my point is clear. For many, it's not "the same thing" as long as a game can physically be played on your PC. If you don't care about any of the above, that's fine. But don't act like people who do care are just silly fanboys if they do.
Edit: I also forgot about the importance of Steam Refunds for consumers, and how Steam Workshop made discovering and implementing mods easier than ever before. I told you I could go on! XD But I felt these two in particular were too important not to mention.