Why does these two threads not run synchronously?

Why is the first thread blocking the second thread from printing?

Who says it's blocking? Maybe starting a new thread takes long enough that the first additional thread (running in parallel with the original thread) finishes its printing (to stdout's buffer) before the second additional thread arrives at the point of trying to print anything.

On the other hand, POSIX does specify that the stdio functions perform operations on streams as if there was a lock associated with each stream that a thread must obtain upon entry to the function and releases upon exit. Thus, the first thread may indeed be blocking the second via the lock associated with stdout.

Moreover, when a thread unlocks a lock and then immediately tries to re-acquire the same lock, there is a high probability for that thread to succeed immediately despite other threads contending for the lock. As a result, when an entire loop body starts with acquiring a lock and ends with releasing that lock -- as is the case in your code for the lock associated stdout -- it is common for one thread to be able to monopolize the lock for many loop iterations.