Because OP, Grass-types get:
>Poison Powder
>Stun Spore
>Sleep Powder/Spore
>Leech Seed
>Synthesis
>Aromatherapy
Since Pokemon is ostensibly an RPG, you can view the three Types as "classes" of a sort.
Fire is all about damage; there is only two Fire-type moves that do not deal direct damage (Will-O-Wisp and Sunny Day) and even then, WoW deals burn damage and SD boosts Fire-type moves and stats. Fire-type moves also generally only additionally cause Burn or change the user's stats. Fire is pretty clearly about dealing damage in battle. Barring few exceptions, Fire-types are glass cannons as well, further compounding their role as "damage dealer".
Water-type is more rounded; the majority of Water moves additionally do something special (Aqua Jet is priority, Waterfall has Flinch, Scald has Burn) a lot of the time, and along with Normal have a lot of HM moves (Surf, Waterfall, Whirlpool, Dive). Water is the utilitarian type, capable in many situations and useful outside of battle. Water-type Pokemon have a wide variety of roles to further their utility; you have both sweepers and tanks, and make use of their weather the best.
Grass-type is the specialist class. Not only do they get a variety of status moves (the powders and spores), they get healer roles (Aromatherapy, Synthesis, Ingrain) as well. You also have buffs and debuffs in the form of the cotton moves. Their moves very often have additional bonuses, most notably the HP drain moves. They are the best users of their environment, having the highest amount of abilities related to weather and the ability to benefit from Grassy Terrain, as well as the ubiquitous Solar Beam. This variety is balanced out by their lack of other move types, barring a few dual-typed Pokemon.
Basically OP:
>Grass = Status
>Fire = Damage
>Water = Utility
