If a sale of a good over $500 requires a writing, why do customers typically not sign a contract when they purchase a computer at a retail store for more than $500?

Any contract that involves goods worth more than $500 should be in writing except when the buyer immediately pays and receives the goods. That means that the whole transaction is over, so there is no reason why written contract be made.

E.g. I purchase an expensive watch, and immediately after I pay for the watch I can take it with me. A written contract would just make the process unnecessarily long.

Written contracts are required when the transaction is not finished immediately either because the goods aren't delivered immediately or they are not paid for immediately.

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