Because sanitization is not easy. You can visually spoof user names with Unicode, e.g. this Unicode: "аdmіn", is not really same as this ASCII: "admin" [0]. Bot

Because sanitization is not easy. You can visually spoof user names with Unicode, e.g. this Unicode: "аdmіn", is not really same as this ASCII: "admin" [0]. Both would create two separate accounts without input limitation. There is more variety with available alphabets [1]. [0] https://onlineunicodetools.com/spoof-unicode-text?input=Nothing%20happens%20unless%20first%20we%20dream.&insert-zero-width-spaces=false&spoof-punctuation=false&spoof-spaces=false&preview-spoof=false https://onlineunicodetools.com/spoof-unicode-text?input=Noth... [1] https://qaz.wtf/u/convert.cgi?text=admin https://qaz.wtf/u/convert.cgi?text=admin