why FTP use 2 different packets for PASS and USER

As I use Wireshark to capture packets, i realize that the USER packet and PASS packet is sent in different packets. Is this some sort of standard? Why don't we combine username and password in 1 packet?

I was trying to find a lot of sources and stuff, but couldn't find anything related.

tldr;

USER and PASS commands are treated as separate requests because they are logically distinct steps in the authentication process that the server must handle independently.

The answer is in RFC959

4.2. FTP REPLIES

Some commands occur in sequential groups, such as USER, PASS and
ACCT, or RNFR and RNTO. The replies show the existence of an
intermediate state if all preceding commands have been successful.

Note that "sequentially". Now in 5.4, we have the sequence of commands and replies.

For the LOGIN:

 5.4.  SEQUENCING OF COMMANDS AND REPLIES

 This listing forms the basis for the state
 diagrams, which will be presented separately.

  Connection Establishment
       120
       220
       220
       421
  Login
    USER
       230
       530
       500, 501, 421
       331, 332
    PASS
       230
       202
       530
       500, 501, 503, 421
       332
   ACCT
       230
       202
       530
       500, 501, 503, 421
   CWD
       250
       500, 501, 502, 421, 530, 550
   CDUP
       200
       500, 501, 502, 421, 530, 550
   SMNT
       202, 250
       500, 501, 502, 421, 530, 550
          

Note that each command has its own reply list from the server, and how USER and PASS are sequential commands.

In short, FTP's structure naturally leads to this: USER and PASS commands are treated as separate requests.

The server handles these commands sequentially and independently, meaning that the commands must be sent in different packets.