The biggest barrier of contributing back is that it takes a lot of time and effort (even for small projects) which you are not obliged to make. A lot of the time the driver for contribution is only the person's internal belief that he/she has to do it and honestly sometimes even when you want to contribute back, you don't have enough time or energy after 8+ hours of work.

It can be a big difference if the library author allows contributions much-much easier to make.