Because American men average around five nine and given the average lean muscle mass needs on that frame size, something within 60-90g (which is a huge range!) will work for most American men.

Like if someone is a 6 foot 10 body builder, they know their needs.

Also the suggested range of g/kg ranges from .8g/kg to 1.2g/kg, which is also a huge range, but that is primarily for building strength, not maintaining.

Given the goals here are "rough guidelines on eating healthy", I'm fine saying most men should aim for 60-90g of lean protein a day. That isn't exactly a hot take.