Why?
Iâm sure itâs true!
Even when using a stylus, some people prefer to draw with a brush that doesnât change with pressure. Look at that one:

This is a study from @activity, the original work is from Akai Sashimi. The original artist also uses a straight brush. You can see their other work here: 蔀äșăăăż (@sas_akai) | Misskey.io
Link taken from Deevad on activityâs study post:
It is a good feature to have, especially for those who do need it, but artists are all different so they all have different needs. I also guess some prefer to do animation with a fully straight brush, as a line that would look different from frame to frame would be annoying to some.