Diversity doesn't make everything a Grey mass of a blob.
True - if they kept what made them special. But FR files off all the serial numbers and doesn't provide nearly as interesting connections to the world, making it much less vibrant.
Not all races go in FR, Warforged don't.
Here's the Warforged entry from ForgottenRealms wiki.
Warforged were a rare race of humanoid constructs created through powerful rituals. Warforged averaged 6′‒6′6″ (1.8‒2 m) and were made up of bulky (albeit resilient) metallic or stone bodies that weighed roughly 270‒300 lb (120‒140 kg). Extremely powerful and durable, warforged were able...
And a great example of what I was talking about above.
In FR, Warforged are rare race of humanoid constructs.
In Eberron, Warforged were created by House Cannith and sold to all of the nations as living weapons in the Last War. They weren't even seen as slaves, but as possessions. The Last War ended with the Treaty of Thronehold which recognized the Warforged as people - but also ordered the closing of all the creation forges so it is a slow genocide. People in Eberron who see Warforged can be reminded of the horrors of the Last War, can treat them as object or rejoin in their new liberty. Warforged can accept this, or look to rebels like the Lord of Blades who is attempting to restart creation forges in The Mournland.
See the difference?
The real world is a "Kitchen Sink" and that is makes it special, instead of one note and unevolving.
No, the real world is like I described Eberron - each race has it's own place and multiple cultures. There are different cultures of elves, different cultures of halflings - each that has a place in the world.
If the real world was a kitchen sink, we'd have dozens of alien races and all sorts of things from outside a single world.