Why does pwsh.exe in combination with Start-Process not accept a script block directly?

Why does Start-Process powershell.exe { Read-Host } work, but Start-Process pwsh.exe { Read-Host } does not?

I'm aware of the -ArgumentList switch of Start-Process, but it makes things more complicated when it comes to escaping quotation marks:

PowerShell 7.0.3 and 7.2.0-preview.3:

Start-Process pwsh.exe -ArgumentList '-Command "& {
    ''Hello World.''
    Read-Host
}"' -Verb RunAs

PowerShell 5.1:

Start-Process powershell.exe { 
    'Hello World.'
    Read-Host
} -Verb RunAs

On the other hand, when creating a new PowerShell session (without Start-Process), it is still possible to use a script block:

pwsh.exe {
    'Hello World.'
    Read-Host
}

Am I missing something here or is there a better/different approach which allows a script block to be executed parallel to the rest of a script?

pwsh get a filename by default in arguments.
If you try to run it in CMD you will see this:

CMD> pwsh { 'Hello!' }
The argument '{' is not recognized as the name of a script file. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.

Usage: pwsh[.exe] [-Login] [[-File] <filePath> [args]]
                  [-Command { - | <script-block> [-args <arg-array>]
                                | <string> [<CommandParameters>] } ]
(.. other help from pwsh ..)

It meant if you want to run some command you must write -Command { ... } in arguments.

But you can write more short command for run pwsh like this:

Start-Process pwsh.exe '-c', {
    'Hello World.'
    Read-Host
}

1. -c is shorter equivalent for -Command
2. Script block will converted to valid string in arguments