Because a proactive meat shield and a reactive meat shield are both meat shields. They both have access to an ability that puts you top of the hate list instantly. They both generate threat through a hate rotation, they both have damage reducing abilities, they both have an "oh-shi..." temporary 'invulnerability'. Evasion or debuff tanks are very different animals.
There are no support classes.
There are no gambler classes.
No pure crowd control classes.
Yes, it will mean certain classes are left out for certain content. The trick to dealing with it is a) level more than one class. b) not have one type of content provide the out-and-out BIS equipment.