Several reasons.
-PuTTY pscp allows raw passwords on the command line, or from a file. OpenSSH is unreasonable in refusing to do this.
-Scripting can adapt to a .netrc easily; OpenSSH will never do this.
-Modern OpenSSH is a nightmare when using legacy crypto, while pscp is fluid. There is nothing wrong with hmac-md5, and no reason to refuse it. I will take PuTTY or dropbear in a heartbeat over these burned bridges and workarounds.
https://www.openssh.org/legacy.html
-pscp does not link to dozens of libraries as ssh/scp does, so it is easier to build with less dependency. The ldd output of ssh and scp on rhel9 is 23 libraries, while PuTTY is 3 [package obtained from EPEL].
-pscp strongly leans to SFTP on the backend and can be directed to use it exclusively, so there is no ambiguity.
-Using pscp with a retry on fail is much easier than sftp -b.
-The wacky cipher control on rhel8 does not impact the PuTTY tools.
That is an extensive list.
> -PuTTY pscp allows raw passwords on the command line, or from a file. OpenSSH is unreasonable in refusing to do this.
You can use `sshpass` to force it through a command line argument. However, arguments can be viewed by any process through `/proc`, `ps`, etc. It's pretty reasonable to not support exposure of the password like that, especially since you can force it through using another tool if you really, really need to.