One reason is that different distributions of linux do things slightly different. Would it be better if there was only one linux os? For some devs of third party software, probably, but diversity and freedom to fork software has been good to linux, and no one could decide what everyone else should use anyway.
So, each distribution takes the available software and package it to fit their distro specifics, and those packages go into their repositories. The benefit of using official repositories is that someone has gone through the trouble of making sure it will work on your system safely. There's accountability and hopefully a bug tracker etc. When you download from a random website you have to trust them instead. Then... you have companies working outside of this model, usually they provide a flatpak or their own third-party repositories. Then you get all these extra steps, but it's not how most distros prefer to handle software.