Because nobody in their right mind picks a nakedly ambitious person for VP. You would have to look over your shoulder all the time. Trump is no different and Ohio is a must win swing state so Kasich makes sense. It would also help rebuild bridges with the party.
McKinley picked Roosevelt, who was the most nakedly ambitious, shamelessly publicity seeking politician of his day. People forget what an obstreperous fellow he was, and how many enemies he had in the party. With modern media he could have been very Trump -like, though much more erudite. And a better shot.
Trump is an old man, the office is a killer, and energetic though he is, I don’t see him doing eight years.
Kasich may bring Ohio, maybe, but he’s a lousy campaigner.
In most of the country he’s that other guy, whatsisname.