Because you wouldn't get your money back at the end of that 50 years. If you sold it after 35 years, you would essentially be selling a 15 year contract to own the apartment, which would likely be factored into the resale price.
With that said, 50 year property rights are likely a stepping stone to something "better" since no one has hit that point yet, so the laws could change before then. However a lot of other things could change before then that might work against you (e.g.increase in corruption, attitudes towards foreigners could sour, property rights policies could revert back to less rights, etc).
