They've been at it so long they now don't realise that 'people who menstruate' sounds utterly bonkers. And deeply offensive to almost all women.
Most people immediately jump to other examples - 'what next, mothers being called people who give birth?' It instantly pissss them off.
Then the inevitable 'are men ejaculators, then?' And realising how utterly absurd that sounds and how it would never happen, is rather enlightening to all concerned.