The same reason why there are many political parties, or restaurant brands, or even games: everyone thinks that their way is better, or are trying to improve on something that they like.

I get that it can be daunting for someone wanting to start, but just like with other topics you should start with something simple to dip your toes and move on from there. My personal recommendation has been Linux Mint, it has sane defaults, works out of the box for most cases and it’s popular enough (and based on another popular distro) which makes getting help easier.

Trying to pick your distro from the sea of available options might feel impossible, but 99% of the stuff is the same, so picking something that works out of the box for you is a great start, and you can learn what you need to make a meaningful choice afterwards.

Let me give you an example on the sort of differences you can expect: do you think that updates should be delivered as soon as possible or leave some delay to catch bugs/issues? If a delay how big should it be?; How much stuff should come installed by default with your system? Should you get a bare ones system to build up or something at least functional for everyday tasks to adapt?; Should you be able to edit your system files or should they be locked so it’s harder to break stuff?. These sort of questions might seem pointless, but they’re the core differences between Linux distros, and if you’re answer to them is I don’t know/care anything that works out of the box would be good enough until you develop an opinion on the matter.