It seems like turkey wants to take the land from the Kurds and settle a bunch of refugees there.
Call it the Occupied Territories model.
'Syrian Kurdistan' has been, all things considered, a fairly successful autonomous governate. But all told there are only around 2 million or so Kurds there. Across the border in Turkey there are more like 12 million Kurds (which is roughly half the Kurdish population in the region). The last thing Erdogan wants is to see the Kurds under his political control taking inspiration from the success those people have had in Syria and deciding that maybe they need to push a bit harder for autonomy within Turkey.
The problem is that no matter how deep you extend the buffer, it will still be the border. So you fix the glitch by eliminating the autonomy. Or the people, whichever comes first, I guess.