Pakistan already struggles enough with the tribal district due to it being so different to the rest of the country (including the rest of KPK) because of the very nature of it being a tribal society. I don't think it could cope with a different country being added onto it, especially one that is so different like Afghanistan.
Even at the basic level in terms of ethnic rivalries, they have a different sociological character in Afghanistan and Pakistan. Afghanistan has a very premodern idea of ethnicity - it is very different to a similar war torn place like the ex-Yugoslavia, which is something NATO did not realise when going in. For example, it would be rare to meet any Afghan who, whilst hate an opposing (or all other) ethnic groups, would not at the same time identify as a proud Afghani or one that would primarily identify through an ethno-national lens. Pakistan has a very postmodern idea of ethnicity, which is the dominant idea of ethnicity in the world - ethnic seperatism is a key component of the ethnic sectarian tapestry here whereas it isn't so much in Afghanistan.