This is a question which has been asked often over the past years. And despite there being posts on several Social Media platforms, I have decided to research this topic myself and include my own take in the matter.
A user on reddit said that due to Sonic’s base shape, people tend to draw their own characters based off him, which makes sense as he mainly consists of circles and lines attached to said circles.
Seeing as Sonic has been present for so many years, it is also a given that by now a lot of people, especially those who have been Sonic Fans since their childhood, have created their own characters to fit into the community.
A lot of those fans created their characters due to social isolation or bullying directed towards them, as a coping mechanism.
Creating a Fan Character itself is not difficult at all, which is why people keep making them, because even people who cannot draw as good as others can easily look up OC makers and edit the base with the features given until they see it as fit.
OCs/FCs also caught the attention of several public sites and Sega itself, to the point where they implemented a feature to create your own playable character in Sonic Forces.
And while this community has a big community including fan characters, it is not certain if it truly is the biggest community with such.