Why do dinosaurs look so similar

I don't know if it's just me but they look so similar. For example, many therapods have the same body shape. Their differences are size, location, time period and certain distinct features (such as crests and feathers).

is this linked to evolution? or else what?

You may not be a paleontologist but answer as well as you can

Form follows function. Theropods are similar because they are bipedal meat-eating reptiles. There are also some bipedal archosaurs, such as Postosuchus, which are not considered theropods or even dinosaurs, but they nevertheless look similar to theropods. That said, there are many variations on the theropod body plan, and indeed some of them look sort of like birds, hence names like Struthiomimus ("ostrich-mimic") and Pelecanimimus ("pelican-mimic). Of course, there are some dinosaurs that don't look very much like other dinosaurs, for example sauropods with their extremely long necks and elephantine bodies. Then there are the bizarre ceratopsians, the best known of which is probably Triceratops. Other well known dinosaurs that look nothing like theropods include the well known Stegosaurus and Ankylosaurus. In contrast, the living lizards look more similar to each other than the dinosaurs do.

They look similar because we do not actually know what the looked like, in most cases we do not even have complete skeletons, thus most of the depictions of dinosaurs are artists impressions, society has developed an ideal dinosaur look and hence most artists depict them that way, physical similarities are not that great, people forget that there were thousands of types of dinosaurs, some had similarities just like most antelope share similarities, the comparison to humans is redundant as humans are one species while dinosaurs are a multitude of species, a more accurate description in that sence would be to say why do mammals have similarities

I will answer your question with another question: why do humans all look the same?

If you think about it the difference between african and albino? (Not meaning to be racist) colour.
Between native and american? Facial features.

Again, I'm not trying to be racist.