Pakhtunkhwa or in ancient days known as “Pakhtuka” taken directly from the Greek Paktuike has been mentioned by Herodotus in multiple verses.
Pukhtunistan has been called by Afghans as “The land of Roh” It is and was the land of their Soul for centuries. Even when Afghans who have settled in Delhi, who served under the court of the Khalaj (Ghilzai) and Sori (also of Khalaj confederacy) , as well as the Moghul court mentioned their ancestral & spiritual land.
Mahmud Ghaznawi when speaking to his son about his territory clearly made a distinction describing and forewarning his son about the territory beyond Ghazni as the land of the Afghans. Even the British named this area as “Afghania” The people there are probably some of the most fiercely independent Afghan tribes of them all. Alexander the Great took an arrow to the leg in this region.
When the Ghorids, Ghaznavids, and even Persians massed their armies to invade India, Pukhtunkhwa was a virtually unlimited treasure trove of troops. It is right on the border of the Persian world and the Indian world surrounded by mountains and deep valleys, gorges, deadly cul-de-sacs, 8,000ft+ high mountains and is perfectly suited for Afghans. In the classical days Afghan Kings would travel to Peshawar during the winter because the weather was milder, while during the heat of the summer they’d travel to Kabul to escape the humidity.
Those valleys and passes have been guarded and controlled by these Afghan tribes since time immemorial. Which is probably exactly why the British chose to make the border right at these passes, and overlapping in territory therin. The Afghan culture of extreme hospitality is practiced here as well. They also share in the same rural-tribal code of Pakhto aka Pashtunwali.