Because as far as you know, the "rough implementation" only works in the happy path and there are really bad edge cases that you won't catch until they bite you, and then you won't even know where to look.
An open source project wouldn't have those issues (someone at least understands all the code, and most edge cases have likely been ironed out) plus then you get maintenance updates for free.
An open source project wouldn't have those issues (someone at least understands all the code, and most edge cases have likely been ironed out) plus then you get maintenance updates for free.