Well, it is just yourself you want to convince, sure. But for others, you sort-of have to have demonstrated skills and that is where the PhD comes in. After all, you cannot just call yourself a surgeon or a pilot or the like either. Well, you can, but it will be meaningless. Although almost all self-styled "scientists" have no clue what they are doing and due to Dunning-Kruger are in addition unable to see that. The scientific method is a bit more involved than most people realize and that includes some pretty smart people. For example, "following the scientific method" is necessary but not sufficient. And even that part is actually pretty difficult to do and takes a lot of different forms. A single life is too short to find out all that by yourself.
Hence assuming that somebody without a PhD is not a scientist is a pretty safe bet.