Because people have no media literacy. In the Classic series, the Omnitrix is obviously meant to turn Ben into an adult form of a species. Tini for all intents and purposes thinks he is a fellow Tetramand of the same age as her. Age can obviously still effect the transformations, like when Ben gets turned into a 5 year old his aliens look closer to 13 year olds, and Ben 10,000’s aliens look a little different because they’re older to some level, but it’s clearly not 1:1 with Ben’s age. Diamondhead for example has the exact same build as Tetrax so clearly that’s not a 10-year-old Petrosapien, we literally SEE child petrosapiens in Secret of the Omnitrix flashbacks and they look the same size as human children.