How come my mcbook pro 2020 is not able to use my imac 2020 as a dual display with the thunderbolt 3?

Imacs were never intended to be used as external displays. If you need an external display for either your MBP or iMac then you need to buy one. This will proved to be much more simple (and likely less expensive) then attempting to connect 2 devices that were never designed to be connected as displays for one another. The good news is external displays have come way down in price.

Also, I'm not sure if you know this but we are all end users just like you. Apple is simply our host and they do not read or participate in these forums. If you have feedback for Apple please provide that at www.apple.com/feedback. Finally per the Terms of Use we all agreed to, we cannot speculate on Apple's policies or products (past, current or future) in these forums.

In order to use an iMac as an external display for another Mac, it must support Target Display Mode. That option was removed with the introduction of the 5K iMacs back in 2015. I believe it had to do with Apple using a custom display driver to drive the 5K displays. Apple was quite up front about this when the 2015 iMacs (and later) were introduced.

Specifically, the ability of any Mac to access and use an iMac as an external monitor via Target Display Mode was removed earlier this year.

It has been over SIX years since any iMac could be an external monitor for another Mac. Only a small sub-set of iMacs made starting with the Late 2009 models and ending with the Mid 2014 models worked; 2009 and 2010 iMac had to be 27-inch models.

Because that feature was deprecated after macOS 10.13, High Sierra.

It is no longer supported.

Because the iMac is a computer, not a display.

If you need a second display for the MacBook Pro, then you will need to buy and use an external display.