How much does it actually cost? You set up a website. You put ads for chocolates and flowers on the site so you can recover your expenses. It's not like you have to build a state of the art fab. It's basically a messaging app, which is the sort of thing individuals have built as a side project.
No, it won't be worth billions, that's kind of the point. It should be possible to do it efficiently so that the operating costs are low and it doesn't need to make billions in order to be sustainable and outcompete the incumbents.
As to why I don't do it, maybe I will -- but I wouldn't object if someone beat me to it. Because it still requires some combination of time and money, the amount of which can be feasible without being zero, and it's quite likely that of all the people in the world, someone who isn't me is in a better position to pull it off before I do.
Moderating it is spam filtering. You limit signups to IP addresses in the the country of your target market and then ban signups from any IP address that tries to send an excessively large number of messages or has an excessively high block rate.
At that point you're down to real users who are jerks, the solution to which is the block button and a message sorting algorithm that takes into account how many times it's been used against someone.
