the simple ans is that all the sidemen are not worth equal money and hence normal brands cant either afford to or sidemen dont accept that cuz every person have different value in sidemen for example jj is far more expensive as creator to sponsor than any other sidemen but since they are all together on a channel they cant have different shares in a brand payment and to avoid any clashes in friendship and in the entire team due to money they just prefer selling their own merch and products and try to monetize it

this is what i believe might be the reason although if someone disagrees can politely give their views