The whole point of flight mode is to prevent the phone transmitting...

I think it’s more an enormous band-aid because cell networks were too afraid of the infrastructure effects of devices traveling at 500mph, perhaps especially with poor signal quality.

If it was a safety issue, wireless devices would be outright banned.

Dangerous chemicals, weapons, and such are not allowed. They can’t just be “off” in a box because accidents do happen.

Think nobody forgets to turn airplane mode on?

At this point, I’m really not sure why we keep up the ruse.

We live in a bizarro world where a small bottle of water is a bigger security threat than an RF transmitter.

And yet it's a reasonable question, because who's to say that it isn't slightly more complicated? Almost everything else is after all:

“Federal Aviation Administration (FAA) made a recommendation on the use of electronic devices in airplane mode—cellular telephony must be disabled, while Wi-Fi may be used if the carrier offers it.”

--https://en.wikipedia.org/wiki/Airplane_mode