Why does Photos upload all your photos again when you migrate from one Mac to another?

It actually isn't uploading but just comparing like you mentioned. It still is a lengthy process as it has to read each photo entry in the local library, search for the same photo in the iCloud library and read all of its data and then compare.. Repairing the library by opening it with the Option+Command keys held down will trigger the same "recertification" of the library.

As far as iCloud Photos Library is concerned, you just added a new Library. In order to ensure all your (potentially) new images are copied to iCloud, it uploads them to compare to what’s already stored. Yes, it’s a very slow process, leave your MacBook powered overnight. This should not result in creating duplicates.

Speaking for myself, I’d rather be safe than sorry when it comes to my photos. It doesn’t cost me anything for iCloud to do a thorough job of comparing, rather than to loose images just because their most basic and limited metadata are same as other images.