To give a somewhat more serious answer, people don’t come here to see people expressing themselves, they come here to get their support questions answered.
You can express yourself in your own blog and other social media, which have their own sites and such. Expressing yourself here simply adds unwanted clutter to threads, causing frustration to users.
Whilst your signature might just be a simple link, some people might want to add 30 lines of text and art, which would then appear after every single post they make. Frankly, we can do without that. It’s easier to have a blanket rule about it than to attempt to police individual signatures and then be distracted into rules-lawyering when someone thinks that their signature is “ok”.
Well, indeed what you wrote is right but somehow you can limit the characters in signature isn’t it? Beside that, you can lay down thousands of rules but you cannot stop abuser but control the damage control. Likewise it work the same for the content of the posts. Don’t you policing every content of the posts including the designation section of the forum.