These are not $30,000 cars that one can replace within 2hrs with a phone call to a dealership. These are $100,000,000 airplanes that take can take multiple years to replace.
If I was a lessor and you wanted to buy the car outright that you were leasing from me, sure, I'd let you have it for the remaining balance and a nominal fee for early contract termination. I've got no appreciable worries because I can take the money you just paid me, go buy a new car, and lease that to someone else (if I had some reason to think I couldn't get a new lessee on a new vehicle, I would be more hesitant or charge a larger fee, but that's usually not the case in that business).
This is even less comparable for the auto leases that most people deal with (as opposed to a fleet leasing company that most businesses deal with), which are usually directly from the manufacturer (or their associated financial arm) -- they're in the business of selling cars, one way or the other, so if you want to buy it outright that's great for them, so they're not about to charge you much (if anything) for it.
But if I am leasing a plane to you, I'm betting on making $1M a month (or whatever) for the next 10 years. If you come and want to buy-out the contract, I've got to go order a new plane to replace the one you want to buy from me, and I'm now stuck without a plane to lease for a solid 2-3 years while I wait for a new one to roll off the line. That's a LOT of income that I'm not going to be getting anymore. I'm in the business of leasing planes, not being some sort of dealership for a manufacturer (and if I was, I'd be charging you a significant mark-up over the price I paid to the manufacturer, essentially the same as a lease termination fee). I need planes out on lease making me monthly income. Sure, maybe 70% is too high, it makes more sense for the amount to change based on the remaining term of the lease compared to the production queue (time for lessor to replace) -- 70% is appropriate for a 2-year lease as most of that time (if not all of it and more) I'm no longer making a dime that I was expecting to because I cannot get a replacement plane that quickly. But 70% is far too high for a long-term contract as the lessor's costs to terminate the contract are nowhere remotely near that much and such onerous terms on lease termination would never hold up in court as you cannot charge appreciably more than actual foreseeable damages.