Why does a background Thread block rendering?

First of, I'm well aware that Unity is not thread safe, but I am doing some background processing in a thread which is later used in Unity in a thread safe way (I'm making a web request for data I need during runtime).

I am starting the thread like this (slimmed down for clarity):

 public IEnumerator StartThreadRequest()
 {
     System.Threading.Thread t = new System.Threading.Thread(() => 
         {
         using (System.Net.WebClient client = new System.Net.WebClient())
         {
             Thread.Sleep(10000); // long running web request simulated with sleep.
         }
     });
             
     t.Start();
 
     // both the while loop and the below t.Join() will block rendering (tried them both separatly).
     while (t.IsAlive == true) {
         Thread.Sleep(2);
     }
 
     t.Join ();
     yield return null;
 }
 
 // The above method is called from the Start method in a 
 // class implementing : MonoBehaviour as a coroutine

 StartCoroutine(StartThreadRequest());

Why does this block rendering, and how do I let the rendering continue.

When this is run the game basically freezes.