Why do airlines retire the flight numbers of planes that have crashed? Is that just superstition?

Probably has to do with inventory, accounting and other record keeping.

Also, hopefully, out of respect.

“Irrational” as in strictly not rational, yes. “Irrational” as in promoting extreme anxiety, not so much.
I think it is likely more of a marketing decision. Any negative association with a product and/or brand translates as a loss of business.

It makes sense since the references to a tragedy are often to the airline and flight number. People refer to “Malaysian Air 370” or “Pan Am 103” or “KAL 007”.

It’s like the Hurricane Center retiring names of catastrophic storms. No more Katrinas or Hugos or Andrews.

I’m thinking that it’s mostly out of respect rather than anything else.

For instance, NASCAR didn’t use the number 3 for years after Dale Earnhardt Sr. died. It was never really an official policy, but after Earnhardt’s death the Goodwrench team renumbered their car to 29, the only time the number 3 was in a race was when driven by Dale Junior, and IROC renumbered the 3 car to 03.