Because it's one of the most annoying and unintuitive things the GNOME desktop has for anyone that isn't a power user. Almost every single user I've shown GNOME to was surprised or bothered by this being the default instead of the usual scrolling you'd see in Windows.
I personally dislike this feature a lot, and it's very common for me to middle-click paste accidentally, even after years of using Fedora Linux as my one and only operating system across all of my machines. I've previously used a Firefox extension to override this default, but was bothered by the fact that other applications would still just middle-click paste.
Not everyone is a power user. Not everyone has the same workflow as you. Decisions like these have to be taken based on what the target audience for a desktop wishes. Arguably, GNOME is absolutely not for power users (just take a look at how similar it is to the macOS desktop environment to notice that).
I personally dislike this feature a lot, and it's very common for me to middle-click paste accidentally, even after years of using Fedora Linux as my one and only operating system across all of my machines. I've previously used a Firefox extension to override this default, but was bothered by the fact that other applications would still just middle-click paste.
Not everyone is a power user. Not everyone has the same workflow as you. Decisions like these have to be taken based on what the target audience for a desktop wishes. Arguably, GNOME is absolutely not for power users (just take a look at how similar it is to the macOS desktop environment to notice that).