It's a vicious cycle. If they disabled upvoting yourself, abusers would just create new accounts to vote for them. Then when you police having multiple accounts, abusers will collude with others to create accounts upvote you and split the profits.

There is big money at stake. 1000s of dollars may not be much to you, but think of what it means to people in poor countries. Whales buy steem power and create sweat shops where the poor work 16 hours a day upvoting the whale and in return get paid pennies and die young. Allowing upvoting of yourself may not prevent sweat shops, but at least there is not much point in creating sweat shops when you can just upvote yourself.