They earn it. Being worth billions doesnt mean having it in the bank account. That is the value of heir assets and investments. This is why their value can swing so wildly.
They don't earn it, to earn something means you have to have done all the work yourself, if you've done 1/2 of the work & someone else other 1/2, other person should earn from it also.
Work is only one issue. What about the initial investment? You dont think the owner should be compensated for what they did? Then no companies will come, and no one will have work.
Capitalism doesn't work this way at all. It is one sided, it is centralised decision making and have extremely unfair rewards distribution (and few other negative things).