That's a really good question!
I just ran tests and found indeed it needs 7.4 to work, but I can edit the config under 7.4 and then change it back to 7.1 and the store will work.
Unfortunately the other software seems to have stopped updating and most of my administrative systems were built around it, so I can't go to 7.4 until I can solve that.
But why does Zen Cart documentation say that 1.5.7 will run on PHP versions from 5.6:
PHP Version
The following list shows the PHP versions compatible with each version of Zen Cart.

Zen Cart v1.5.8 is designed for PHP 7.3 through PHP 8.2
Zen Cart v1.5.7 is designed for PHP 5.6 through PHP 8.0