First off, Psychic doesn't resist Grass, nor is Psychic super-effective against Grass. If a Psychic move is doing super-effective damage to a Grass type, it's probably due to the Pokemon's secondary typing.
Flying type in Pokemon is pretty much Air or Bird type. Many types of birds eat seeds and other plants, such as Goldfinches, Chickadees, Cardinals, etc.
Poison types are likely super-effective against Grass because of weedkillers and other chemical products.
Ice attacks are super-effective against Grass because many plants wither and die when exposed to extreme temperatures.
Flying, Poison, Grass, Steel, Dragon and Bug likely resist Grass type moves for the following reasons:
- What is a plant going to do to a bird or air-dweller, especially because they need dirt to grow and are confined to the Ground? Birds often nest in trees anyway.
- Poison types likely resist Grass for the same reason- what is it going to do?
- Dragon types resist all four types of common elemental magic; Ice, Fire, Electric, and Grass. Dragons are highly magical creatures, so it makes sense for them to be less affected by powers they themselves can control.
- Bugs are notorious for eating and destroying vegetation.
- Steel is metal. Grass cannot grow on metal. Therefore it will die.
- If you add Grass to Grass you get......grass. Basically, same explaination as Dragon types.
it makes sense for them to be less affected by powers they themselves can control.
This quote also sums it up well.
Pokemon generally tries to be realistic with type weaknesses and resistances. It makes sense that Water beats Fire, and Fire beats Grass, because that's what would happen irl. Birds and Bugs eat away at plants, so that also makes sense. Plants that don't have needles or go dormant in the winter can die if they freeze. And plants tend to not like it when you put toxic substances near their roots. Sure, animals don't like it either, but it only hurts them if they get it into their bodies. An animal can just move, find a different source of water or food. A plant is kinda stuck with whatever is in the soil underneath it.