Why not write it like this? Since you have raised this question, I'm afraid you haven't found that others have written one by one when you look at other people's calligraphy. This can only be regarded as a convention. Calligraphy is not written like this. If we want to explore the reasons, first of all, strictly speaking, one is capitalized, not traditional. Secondly, the usage is different. One is to prevent people from tampering with bills, making false accounts and unifying the numbers of bills and receipts in order to avoid mistakes. Or the kind that appears in the original text of a specific poem, such as "the gentleman teaches, the son learns, and wants to succeed" (Xunzi). These original texts cannot be changed, nor can they replace those that can be written, so the usage of "one" can only be used instead of representing numbers, which is irreplaceable in calligraphy.
Also, seal script, the font is changeable, it is difficult to be elegant. Sometimes it is allowed to replace one with another. Don't change the owner of the right building at other times, just write it and it will look good. I have been writing calligraphy for many years and have never exchanged the two words. In fact, simply saying one and one is enough.