Nope, you're not the only one. In my own headcanon, the professors have access to multiple members of a species, like Elm having a wall of Totodile to hand out if need be. It just seems like something they would be ready for, you know?
But Fransferdy has a point in where the kids have a rivalry going on. If all three trainers have the same Pokemon (or if two kids have Charmander and one has Bulbasaur) it doesn't have that simple "my rival has a type advantage over me!" little bit to add to the storyline/character development. (If the author remembers that there are one/two/more than just the main character and the rival to deal with.)
Plus, maybe people are just afraid they're going to get bad reviews if they do something different than what's seen in the games/anime. Which is a shame, because this is a topic to think about.