You mean ''they'' as Game Freak.
In Pokémon terms, three pairs of games are remakes: Pokémon FireRed and LeafGreen, remakes of the Generation I games Pokémon Red and Green during Generation III; Pokémon HeartGold and SoulSilver, remakes of the Generation II games Pokémon Gold and Silver during Generation IV; and Pokémon Omega Ruby and Alpha Sapphire, remakes of the Generation III games Pokémon Ruby and Sapphire that will be released as part of Generation VI in November 2014. The names of remakes are derived from the name of the original with a modifier attached to the front.
Game Freak is making remakes because the games that are beeing remaked are old, for Game Boy and there are not at selling. Also they want to improve all generation to the best, having better graphics and add all features in the last generation such as all gen 6 pokémons, mega-evolutions, hordes etc..
If Game Freak wouldn't make those games, I don't think Pokémon fans would like and making remakes and improve all their games is the best for Game Freak company for they win more money and players.