Probably because not every private enterprise is looking for a $10 billion contract which is precisely why private entities from abroad have sold their systems to us. Why wouldn't a private Pakistani company not find it enticing to sell arms to Pakistan and make a few hundred million dollars every now and then ?
Besides its not as if Pakistan is the only customer; if you're building quality equipment battle tested by the 7th largest Army in the world and one of the most battle hardened armies of the past decade your products are not just attractive as potentially good ones but are attractive because they've shown their worth in the field - So you can always sell them abroad.
So no I don't think the budget is much of a reason.....what
@HRK highlighted are.