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”.