With a coin like dash that uses a paynode scheme, all you need to do is keep reinvesting and you are guaranteed a greater percent of the stake
There are masternode costs involved, security and technical know-how costs (if you don't have the skills, you have to pay others to do it for you) as well as investment dilution by PoW mining.
This is not proof of stake where you have 1000 coins and they are giving you interest, 'just because'.