Well pretty much no platform can stop people from registering multiple accounts and most platforms don't make it "against the rules" in any way like on YouTube, eBay......etc
So with Steemit it has even less of the ability to attempt to keep people at 1 account. But people have multiple accounts for a lot of reasons. Some of it is to not keep all their eggs in one basket and have multiple accounts with value. Some are to lock down the names like keeping a domain name.
Well, personally am not against keeping more than one account , I just think they should be kept for the right reasons... Well, any thing that has advantages, must have disadvantages too and since we imperfect is difficult for must people to do things for the right reasons instead of always thinking about personal gains