Apple is not intuitive.

The choice is yours. You could drag and drop the entire photos directory, individual albums (folders), or just drag photos. It’s just a copy paste operation. You could also potentially set up some kind of script that would love the photos automatically. That might take some other knowledge

We can't answer "Why" questions.

This is user to user support site, we are all users.....just like you. Apple is not here.

No one here can answer for Apple. Only Apple can explain "why" they do what they do.

Please contact Apple directly.

Contact - Official Apple Support

Other users may offer their opinions if you want to wait to see if anyone responds.

Wesson49 Said:

"Downloading iCloud pictures to an external hard drive. Actually, all data. Apple limits storage space to 5 GB."

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Downloading All of iCloud:

Use my user tip: Getting a Quick and Direct "iCloud Drive-to-External Hard Drive" Download - User Tip

The One Drive step is completely unnecessary. If you set up iCloud for Windows, your photos will sync directly to your PC. Provided your devices are all logged into the same Apple ID, all of your photos will sync and download to your PC or be available at iCloud.com/photos.

This may help: What is iCloud for Windows? - Apple Support

Wesson49 Said:

"Apple is not intuitive.: Why do I have to search the Internet to do anything on Apple Devices? Mostly nothing is intuitive."

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Apple and Web Searching:

So, are all of your devices not working, in their own manor? What's the issue here? How have you troubleshot it thus far? The Internet is a superb place for troubleshooting, but you can use books and talk to others aloud, not digitally. If anything, I'd say Apple is more proprietary, than it is intuitive.

You've added a few steps there. But, yes. you sync with your computer. Then, you can move the photos directly to your external drive. The One Drive step is unnecessary (unless you are using One Drive as a offline storage solution).

Gosh, is it difficult to reason that if 5GB is not enough, you could add more space easily?

Details here: Upgrade to iCloud+ - Apple Support

Such as?

Your free storage in iCloud is limited to 5GB. You can, of course, purchase additional space. And, downloading to an external drive can be done easily by syncing your Photos to your computer.