Does the definition of a “low interest” credit card change over time?

The definition does not change but the consumer sees it changing by the revising of what is considered "low interest" as rates go thru a period of adjustments higher or lower.

Does it ever make sense to get a credit card with a low regular APR?

A low-interest credit card can be a valuable tool to assist you at times. Some cards come with introductory interest-free periods, some up to a year. Always watch for hidden fees. This could be a valuable resource for someone depending on their credit and cash flow situation.(vs. a 0% credit card or paying in full).

If rates are so low in general, why aren’t credit card interest rates lower?

Risk to the credit card company is higher than a mortgage or car loan where they can take the asset, credit cards are basically unsecured loans and more difficult to recoup the balance from a consumer.