I can't answer for others, but for us it is for 2 reasons.
In the U.S., making your own makes you the manufacturer and without proper D.O.T. licensing and HAZMAT regulations in place, it becomes a liability situation.
Other than that, it is for the convenience of being able to give that headache to someone else. I enjoy being able to pass the scales knowing I am not
in violation doing so.
The products we purchase and use also support the businesses that sell them and I believe in that.
Rod