It's because riptides most often run perpendicular to the shore. So you need to go parallel to get out of the riptide section to then have a chance of getting back to the shore unassisted.

They also say it's safer to actually just float and try signal for help than try fight against a riptide (swim against it rather than parallel). Highly unlikely to beat it and it's going to tire you out to the point of drowning

Even though I'm a decent swimmer, I hate swimming at sea. I'll happily swim in Florida lakes (and frequently do) but open sea scares me without all the safety precautions