Not a teacher.
I would replace "...and so as" with "in order to".
You need a "have" between "secure". Otherwise they are making customers safe, which is grammatically correct, but unlikely what you want to say.
check that "favorability" is actually a word in the dictionary. If it is, you can write more clearly:
"Why do the manufacturers or stores spend a lot of money on advertising? I think it is because they want to increase the visibility and customers' favorability of their products, in order to to secure having many customers."