...My question is why do we need so many stable coins when they all serve the same purpose? ...
In my opinion, it's about money that will be gained by creating the stable coins, shortly, it's about Business done by a company by offering their own project (in this case stable coin), with their very interesting offers, promises, advertisement, and also all strengthen that they offer over other stable coins.
and to merge them?
I don't think that it can be because they come from different developers, companies and also projects. They are trying to compete each other by offering their best stable coin.
For me, I will no matter of how they create new stable coins again or not, it's like other altcoins that are coming new almost everyday.
I just need to use some of them that considerably needed and matched with what I need, it's enough.