> Why shouldn't sufficiently-sophisticated symbol manipulation, producing results the same as "conscious understanding" (which seems to be the same as "semantics" as Searle was using it), be identical to "conscious understanding"?

This is what the Chinese Room argument is trying to answer. In the room, there is a human, and the human is performing symbol manipulation according to the rules listed in various books. However, (1) the human has conscious understanding of various topics, but Chinese is not one of them, and yet (2) the outsider is able to send and receive messages in / out of the room in Chinese, indistinguishable from communicating with someone who does understand Chinese. This shows that an entity performing symbol manipulation which results in a Chinese conversation is not the same as that entity understanding a Chinese conversation. To say otherwise would require that the conjunction of 1 and 2 is not possible, but it is clearly possible (or at least this is clear to most people).

There are various replies which address the argument coherently.

The illusionism theory of consciousness denies that the conjunction of 1 and 2 is possible because 1 on its own is not possible. That is, the human in room does not have conscious understanding because no one has conscious understanding of anything (since consciousness does not exist). This theory has not widely caught on because most people think it is just too obvious that conscious experience does exist.

Another reply is that the conjunction of 1 and 2 is not possible because as soon as the human executes enough of the symbolic manipulation rules, they do in fact come to understand Chinese. The trouble with this reply is that it runs up against the strong concrete experience that we have which shows this is not remotely how gaining an understanding of something works.

A few other remarks:

(1) The argument does not show, and is not intended to show, that building a conscious machine is impossible. Searle explicitly thought, and wrote in his original paper, that it was possible to build such machines.

(2) The argument does not show, and is not intended to show, that conscious machines require biological substrates. Searle wrote in his original paper that it was an open question what substrates could be used to build conscious machines, and that it might well be possible to build conscious machines without any biological components.

(3) A reply which says, "alright, the human in the room does not understand Chinese, but maybe the 'room itself' does have a conscious understanding of Chinese" is perhaps interesting. Some people, including Searle, have debated how likely this is. However, it is beside the point of the original argument, which is just about showing a difference between the concept of syntax and the concept of semantics. (And, again, the argument shows that there is a difference by exhibiting a counterexample where an entity is able to follow arbitrarily complex syntactic rules and yet that entity does not have a corresponding understanding of the natural language which the rules implement.)

(4) For some, e.g. people in software development who work with formal languages on a regular basis, the fact that there is a difference between syntax and semantics is incredibly obvious. If you think it is simply obvious that there is a difference, then congratulations, you understand the concepts involved. In that case, Searle's argument is superfluous, and it's not intended for you. His argument is directed at those working in philosophy of mind or related fields who were not familiar enough with the difference between syntax and semantics to understand that there is a clear difference.

edited to add a reply about Point 1 begging the question: just to clarify, the ARGUMENT I posted above is not Searle's argument. It was an arguing I was giving to the previous poster to illustrate the fact that Searle's argument was not equivalent to claiming that consciousness does not exist in humans because individual neurons have no understanding of what the entire brain is doing. My starting point was taking Searle's conclusion as a given, in order to work out consequences from it.