Well carrots and lettuce such as head lettuce (iceburg) are either useless or potentially bad for bunnies. Carrots are sugary, and iceburg is mostly water. Plus, if you look at what pellets were originally created for, it was to raise rabbits quickly (to get good weight on them) for breeders to either make the next generation or supper. Pellets are something created for breeders probably by breeders. And when you only expect to have a rabbit for at most 2 years, a pellet only diet will get a majority of them there at good weights, and at the cheapest possible cost to you. When people first started keeping rabbits as pets, their only knowledge really came from the same breeders they were purchasing them from. It was uncommon to take a rabbit to the vet, or expect it to live more than a few years. Unfortunately, this kind of knowledge tends to stick around. Though the HRS, and now BB has done amazing things towards educating people, programs like 4H continue to view rabbits as livestock, with the main goal being to get them to “harvest weight”.