I thought it was quite simple - why? For the Pakistan Army to retain it's centrality and power in Pakistani society means, it needs an enemy and that is India (just like during the cold war, Americans needed USSR for their military industrial complex) and vice-versa. Given USA/USSR could destroy the world many times over, they did not fight directly but via proxies in mostly 3rd world countries.
Same issue for us - Pakistan cannot fight a conventional war with us; nuclear means assured absolute destruction, so they use proxies (non state actors). Unlike USA/USSR who had countries for proxies, Pakistan cannot afford anything but terrorists. After all they themselves live of the largesse of USA, China, Saudi ... may be even the Emirates ... not much is left after the Army bigshots have stashed away much of it for themselves.