You are getting a price nobody else is
The rule
Your blind is already in, so continuing costs less than it costs anybody else at the table.
When the action folds to a raise and you are the big blind, you are being asked to add the difference, not the whole raise. That is a better price than any other seat is offered, and it is why the big blind defends far wider than it opens.
You also close the action. Nobody is left behind you to raise, so there is no risk of calling and then facing another raise. Those two facts together are the entire reason the defence range is as wide as it is.
What the price does not buy is the rest of the hand. You will be out of position for every street that follows, so the hands that defend well are the ones that make strong hands rather than good top pairs.