Because they're just as correct when you switch the labels.

With two parties both jockey for position in being as close to center as possible while still appearing unique. To leave the center is to sacrifice those voters to the opposition. This is why first past the post voting is stable yet nearly useless.

Democrats consist of actual liberals, marxists, authoritatians, etc.

Republicans consist of actual fiscal conservatives, the religious, libertarians, etc.

Both are unhappy alliances and occasionally fracture and spill one of these constituencies and the opposition rushes to appeal to them.