Why exist limit by 10 characters/account? Exist persons who have more account for playing more character then 10?
You are able to buy more in the shop.
So Kim can run the site probably
Iirc, ten is maximum each person can have free, because the site paid for those ten and as Rei said, it keeps the website up and going.
The site only has one admin and creator, basically homemade, so the site runs through premium memberships or shop purchases. 
What others have said; you get 10 slots for free and then can purchase, or be gifted, items which grant you more from the shop.
As to 'why', because running and keeping the site going isn't free. Kim does a lot of work for us here and it's a labor of love more than anything else. The site is free-to-use but all those character profiles, images we upload, and other items cost server space to maintain. Once a person exceeds their 10 free slots, they're asked to purchase additional ones to help cover the costs associated with the data load.
Of note, items like the Enchanted Silk Cloak and Dragon's Tooth are permanent additions to your account. If you buy the Dragon's Tooth once, you'll always have those 5 extra character slots, unless you delete your account later. Epic Membership, which I'd love to someday get, also grants a lot of extras, on a recurring cost depending which version you purchase. I was gifted two 'Dragon's Tooth' items years ago by a writer who has since deleted their account and whom I've had no way to contact since. Said gifts are still used and appreciated to this day.
As to 'why', because running and keeping the site going isn't free. Kim does a lot of work for us here and it's a labor of love more than anything else. The site is free-to-use but all those character profiles, images we upload, and other items cost server space to maintain. Once a person exceeds their 10 free slots, they're asked to purchase additional ones to help cover the costs associated with the data load.
Of note, items like the Enchanted Silk Cloak and Dragon's Tooth are permanent additions to your account. If you buy the Dragon's Tooth once, you'll always have those 5 extra character slots, unless you delete your account later. Epic Membership, which I'd love to someday get, also grants a lot of extras, on a recurring cost depending which version you purchase. I was gifted two 'Dragon's Tooth' items years ago by a writer who has since deleted their account and whom I've had no way to contact since. Said gifts are still used and appreciated to this day.
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