It's not really about culture. Everyone – OSS or not – wants to write good software. The problem is that the "polish" you mention is what takes up the vast majority of development effort and costs, and FOSS projects just aren't that well funded. Hiring teams of designers, copywriters, user researchers and HCI experts, staffing up customer support teams in different timezones, having dedicated QA for every minor change, overprovisioning servers so the app can scale...none of these come free or easy.