Use iMac 2017 as monitor for MacBook Pro 2021 M1 Max

You can't. Both computers are too new to be compatible with Target Display Mode. Please see the limited hardware and software requirements in this Apple article:

Use your iMac as a display with target display mode - Apple Support

Doesn't make sense to me :(

It makes more sense when you consider what happened with social media and alleged internet "journalists" ove the last two years.

Before COVID we saw maybe 2 questions a month here about Target Display Mode. Now we see a dozen a day. Apple took that external display ability from iMacs in Late 2014 and no one seemed upset. Apple delivered the coup de grace to TDM in early 2020 before COVID hit.

Then COVID came with the need for work-from-home. Starving journalists lacking research skills or an editor with a thimble-full of fact-checking sense belched forth article after article on how any iMac could work as a monitor with any video device.

There is probably more spurious information on the internet today about Target Display Mode than there is about the Loch Ness monster and a flat Earth. TDM has been incorrectly attributed as doing everything from supporting any video device to parting the Red Sea. The flood of post-processed horse feed about TDM was so heavy that even some Apple support employees quoted the internet articles instead of their own company's official support article.

There is no workabout. An external monitor uses less energy, does not require US$60-90 worth of cabling, and allows extended desktop. TDM-supported iMacs do not. Never did.

You can do network screen sharing but most people find it too sluggish:

Share the screen of another Mac - Apple Support

Please refer to Use your iMac as a display with target display mode - Apple Support. The hardware and software requirements are not met.