Sure there is: lack of expertise. Also, unfamiliarity with the code base. For people without programming experience, there's also the opportunity cost of learning a programming language.

The reason open source hasn't taken off with the general public is b/c every time a non-programmer suggests a genuinely useful feature (to non-programmers), the response is: "it's open source, program it yourself." Most often, the easier and cheaper path is to just buy the competing commercial program.