The NFL made a decision many years ago to protect WRs and QBs from getting mauled the way they had been mauled for decades.
The NFL opened the doors to a passing league dominated by passing plays during a period when TV/Streaming contract prices accelerated upward.
So the way to win is throwing the ball downfield, since the rules encourage it. So QBs and WRs are way more valuable than RBs, and the contracts today show that.
The numbers are so massive due to the salary cap, which is driven by what networks/streamers will pay to show NFL games, has increased dramatically recently.
Its all pretty simple. Load up on QBs and WRs , as they determine if you win or lose. And you have the salary cap to pay up for them.
RBs are good to have, but not essential to winning and losing.
Is what it is.