Why do programmers choose to write their own code instead of using code that is already available online?

Read also:

• Why does an older married man turn bisexual?
• Do women like watching men sucking men?
• Did another parent ever tell you something about your child that you didn’t know?
• What is the reason behind the Russian government's negative view on foreign travel?
• Is BPD real or just an excuse?
• Why should you never do drugs? Will this story absolutely shock you?
• Beautiful European women were killed by inquisition but Russia was not Catholic. Is this the reason for a drastic difference explaining why Russian women are the prettiest?
• What is your review of "Regent", episode 5 of Season 2 House of the Dragon?
• What's the difference between “ce”, “ça”, and “cela”, and when do I use each (French)?
• What are the basic human needs according to psychology? What are the consequences of not meeting these needs?