Adult bees can consume nectar directly but nectar is basically sugar water, plant sugar, but is almost completely carbohydrates. Just as every animal needs protein, so do all the bees in the hive. The protein is in the pollen. Bees are extremely efficient gatherers and storers of pollen.
So what’s the problem with bees directly consuming pollen? There are obstacles: a “wasp waist”, the design of a bee’s gut, and the structure of pollen itself. Bee larvae don’t develop the narrow waist of an adult until final metamorphosis. Adult bees can’t easily ingest much pollen through that tube running from the mouth of the bee to the stomach. That esophagus is about half the length of the bee. Even if pollen passes through the honey crop and proventriculus, the bee gut is not efficient at digesting pollen. Pollen has a external component called the exine. Think of it as a protective shell or casing.