while (t++ = s++) ;

You're assigning a char to another, relying on the return value being 0 to detect end of string.

You're performing the copy while also increasing the pointers with ++ in the same expression.

You're using the cryptic ; empty statement to signify nop, thereby confusing newbies.

Etc

Equivalent of a strcpy() there's no bounds checking.

Another reason, in addition to other replies and separate from safety concerns, is that strcpy, memcpy etc are nowadays usually implemented via more efficient compiler intrinsics rather than an explicit loop.