I never learned much about evolution, so please do explain it like I’m 5
In: Biology
In a way they did.
The catastrophe wiped out all land animals larger than a medium sized dog, (and many of those that were smaller too.)
All the species on land that survived were small and often capable of burrowing.
Over time many of these small animal species evolved to be bigger now that there were empty places for bigger animals to fill.
Shortly after the event, some flightless birds evolved to be rather big and carnivorous. Those creatures looked and acted like the recently extinct two legged carnivorous dinosaurs they were related to. Evolution had taken birds and taken them back to their theropod predator roots.
These creatures didn’t quite reach T-Rex size and had useless tiny wings instead of useless tiny arms, but the resemblances was there.
However before too long mammals started to take over that niche. The mammal predators that followed may have filled the same empty niches but acted and looked quite differently.
The absence of large plant eaters meant that predators could not get as big as before and circumstances didn’t push land based plant eaters to evolve to such great size again. Although some mammals like the Paraceratherium did end up getting quite big.
Of course in the water whales ended up breaking records and grew to sizes that reptiles and fish never did.
Generally your answer might simply be that evolution does not have an aim. Similar starting points and similar circumstances may result in similar outcomes, which is why so many animals evolved to look like crabs and why so many mammals ended up with sabreteeth independently, but there is nothing about the size and shape of extinct dinosaurs that made them inevitable to evolve again.
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