You have some point, but look at this way: Let's simplify this by having only one currency - Steem. Actual value of your account is some amount of steems. It's money, digital, but still money. Each hour, some new Steem is created by witnesses (I'm talking about currency, not blogs). 90% of this added value is distributed to current owners of steems. This is why your voting power increase with time. You have been given some money to spent. When you decide to vote, for example for me (yes, your voting is noticed and thank you very, very much - really appreciate it) you are actually adding value to my post. You are giving me tip for my effort. At the end of 7 day cycle I receive salary for my effort - 85% of value of my post. For your effort, you (and other voters) receive 15% of value of the post. By each vote, your power decrease and value of your vote is lowered. This is because you are spending your money. By this way, your investment in posts of other members is limited. That's simplified economy of Steem more or less.
Now - imagine that you can vote without any limits and that all your votes are equally heavy. That means that you can vote 10 or 100 or even 1000 times and each of your votes will be 1 Steem (for example). So, you can add value of 10, 100, 1000 Steems without any limitations. Pretty soon, to maintain that economy, we will run out of Steems and witnesses will need to create much more of them. More money in circulation means that value is lowered and we have hyperinflation... That's practically the end of this little experiment. We killed steem as currency, and all we are left with is another blogging platform without actual value, like Facebook for example :))
I know it is complicated, because it is. I know that instead of inflation of voting power we could have limit of 20 votes with equal power per day, but this inflation system actually works better. With constant inflation from one side and adding value from another side - we have constant effort by users to add some value to steem and as a result we have constant growth in steem value. Simple?!? Noooo... but it's working ;))