There's a fixed amount of (top) witness positions, which will always be filled. Their job is to make sure that the chain is running, and that all transactions are included. This is what they're paid to do. Anything on top of that is voluntary.
Yes, it might make sense to choose your votes according to what the individual contributes to the ecosystem, especially when they're in the higher ranks. But how much additional work besides running the servers can you realistically expect for ~$2k a month?

The comparison doesn't make any sense at all.