Another reason that goes hand in hand with many of the other answers, from the fact that doing encryption well is hard:
Getting encryption right is expensive.
Getting encryption wrong is expensive.
Because properly implementing encryption is such a difficult thing to do that requires oodles of work, from a business perspective it's unlikely to make sense that you implement your own cryptography.
And then you have the risks of getting it wrong, which could include all kinds of negative consequences, including fines, bad publicity, and worse depending on what you're supposed to be encrypting.
