Why aren't starter Pokemon catchable in the wild ?

In game, they are supposed to be "highly rare" pokemon. Real reason is to add weight to your starters decision and make it your special Pokemon.

This shouldn't have been hard to figure out OP

Couldn't you catch the starters in FRLG? I might be misremembering things.

Same reason starters, eevee, lucario and other "special" pokemon have a 7-to-1 gender ratio. They are supposed to be rare.