It was explained to me once that the reason why they don't just allow all the accounts is because the Steem developers and witnesses are afraid of "Sybil Attacks", which is basically when someone gets a bunch of different accounts to abuse (they pretend like they're a bunch of different people to be more influential, or to be able to vote more for themselves).

But honestly, I think they should be more afraid of the paid-vote bots. The paid voting system makes it nearly impossible for new users to grow. IMHO the Steem devs and witnesses should seriously re-examine their priorities.

When I told them that, they basically said they didn't care about my concerns because other people had invested more into Steem than I did, and those people's opinions were more valuable to the SteemIt leadership than mine were.

It's like they don't care if Steem goes down in flames as long as they get their short-term kickbacks.