The reptiles to me look exactly like dinosaurs, their skeleton resemble fossils and they are cold-blooded.
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It is because of how those terms are defined.
The usual definition for dinosaur is cladistically. This means they pick two examples like T-Rex and Triceratops and say a dinosaur is everything in the smallest family tree that has both of the examples.
Basically going back to the last common ancestor of those examples and say that was the first dinosaur and everything descended from it is also a dinosaur.
Grouping things together this way makes a lot of sense, but it leads to some funny results like including birds.
It also excludes some extinct animals commonly mixed in with dinosaurs like flying or swimming reptiles that lived along side them or creature like the large mammal like reptile with a sail on it back Dimetrodon which live long before them. It also excludes crocodiles and lizards and other living and extinct reptiles that share things in common with them.
It makes sense though.
The word dinosaur in casual conversation is usually understood to refer to those creatures that have been extinct for 66 million years and not birds.
Usually the term non-avian dinosaur is used for those if you want to be pedantic.
Technically though a sparrow or a chicken is a dinosaur. Technically a falcon is a carnivorous dinosaur that hunts other dinosaurs as well as small mammals and reptiles. Technically the dodo and the passenger pigeon are examples of extinct dinosaurs.
But that is just if you want to be overly pedantic about things.
The whole idea of grouping creatures together based on common descend, where all creatures that share an ancestor are in a group results in problems in other areas too. For example the different creatures we call ants and wasps are so mixed up with each other, that you can’t create a cladistic definition for only wasps or ants that would not either include the other or exclude some animals we call ants or wasps.
Than there are places were we really want to exclude some animals for a definition because we don’t like the implications of grouping all primates or great apes or chimpanzees together.
Then there are “fish”. You can define fish that way at all.
If you go by clades either fish aren’t a thing or they are everything. Neither is a very useful definition.
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