I never learned much about evolution, so please do explain it like I’m 5
In: Biology
They actually kind of did. Birds are a group of theropod dinosaurs that survived the extinction of all other dinosaurs. And while all of the surviving birds were small, some of them rather quickly again evolved into large, flightless forms that were among the dominant animals of their ecosystems. Now however, they also had to compete against the mammals, which were also evolving very rapidly following the extinction of the non-avian dinosaurs. This fierce competition from mammals might have hindered birds from evolving to become as huge as the largest theropods again.
Still, huge flightless birds would continue survive – including meat eating ones who were apex predators of their ecosystems. Some would survive until only a few tens of thousands of years ago, Late enough that even early humans encountered them. The giant carnivorous birds survived especially long in places with few large mammalian predators, but in the end though they all seem to have disappeared largely due to increased competition from carnivorous mammals, perhaps in combination with disadvantageous environmental changes.
And even though the giant flightless carnivorous birds are now all gone, we do of course still have other giant flightless birds around – like ostriches, emus, cassowaries and rheas. And until very recently the even larger moas and elephant birds, which were sadly driven to extinction by humans just a few hundred years ago. All of these belong to the bird order called ratites, who are de facto rather large, flightless theropod dinosaurs.
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