Because that would be pretty much the only way without hacking, to get 2 equally fast DP connections through TB/USB4. If one DP connection is 4xHBR3, there is no longer enough for a 2nd 4xHBR3 connection or even a 4xHBR2 connection.
To reach the numbers Plugable advertised AND still support 2 monitors on Mac that is basically the only way.
That limitation is why on docks like the WD22TB4 you can only get a 4xHBR1 connection on the TB-outs (under normal circumstances), because the integrated MST-Hub will always reserve a 4xHBR3 connection when possible.
(we have a thread on here where we tried to collect a bunch of details about the internal stuff of TB / USB4: Details about USB, Thunderbolt and dock operation)
Edit:
Plugables FAQs said some things that were contradicting me, but the teardown of the dock (Plugable TBT4-UDZ Dock Review and Teardown – Dan S. Charlton) actually proves me right and shows that Plugables FAQs what the dock should be able to do on DP Alt Mode hosts are just wrong. Boy, do I not trust that brand.