Because there's so damn many of them. The way the game calculates random encounters is super shitty and primitive. You can literally take one step and end up in a battle, win, take another step and immediately end up in another battle.
To put it into reference, the first final fantasy game has a ~25 step minimum before the next encounter. Mother has none
It doesn't help that you have very limited combat abilities for the first few hours of the game, especially before Magicant. Ninten has no offensive PSI, making most battles before you get Lloyd unnecessarily frustrating and/or long