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> The only way for the database system to guarantee there is no conflict is to keep track of every single row your query accesses, even if it is a row passed over by a WHERE clause!

Why is putting a lock on the table itself not an option?

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You're correct that a table-level lock is a useful tool to ensure serializability, but it's also a drastic one since it blocks other transactions requesting a higher access level. The concurrency control system is trying to provide a generic way to both guarantee consistency and maintain a high throughout on a variety of workloads, and locking tables always would diminish throughout for many common workloads. For example, one where inserts and reads tend to occur simultaneously at high volume.

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