Short answer: since macOS 11, the system volume is sealed with a cryptographic signature that only Apple can produce.
No backup tool can write a bootable copy of it back. macOS comes from Apple, and your files go on top.
Modern macOS splits your startup disk into two volumes. One holds the operating system and is read-only. The other holds everything else: your home folder, your applications that did not ship with the Mac, your settings.
The system volume is a Signed System Volume. Apple computes a cryptographic hash tree over its entire contents and signs the root of it. At every boot, macOS checks that signature. If a single byte anywhere on that volume differs from what Apple signed, it does not boot.
That is a security feature and a good one: it means no piece of software, however privileged, can quietly modify the operating system. It also means a copy of that volume made by anything other than Apple is unbootable by construction, because reproducing the signature requires Apple's private key.
Older advice about "cloning your startup disk" comes from a world that no longer exists. Tools that used to make a bootable duplicate now either copy only the data volume, or use Apple's own replication mechanism to install a fresh copy of macOS alongside it, which is a different operation with different limits.
Time Machine included. Its restore does not write a system volume back either. What it does is install macOS from Apple, then put your data on top, which is what Migration Assistant is doing during setup.
So the honest framing for every backup tool is the same: back up what is yours. The operating system is not yours to back up, and it is a free download that has never needed backing up.
Total time on a modern connection is usually an hour or two, most of it waiting for the macOS download.
~/Library, and a backup that skips hidden files quietly leaves them behind./Volumes mount or a folder at the root of the disk.ls /Applications > ~/Documents/apps.txt
costs nothing and saves an afternoon.Not of the sealed system volume. Some tools can produce a drive that boots by installing macOS onto it and copying your data alongside, but that is an install plus a copy, not a byte-for-byte duplicate, and it has to be redone after each macOS update.
Because it was excellent advice for about fifteen years and habits outlive their reasons. Before macOS 11 the system volume was ordinary writable storage and a block copy of it booted fine.
Time Machine has one real advantage: Migration Assistant offers to restore from it during the first-run setup, which makes the recovery a single guided step. What it does not do, and cannot, is restore the operating system from your disk. That comes from Apple.
Recovery installs the version the Mac shipped with or a later one, so a machine is never left without an operating system. If you specifically need an older release, keep the installer itself in your backup: it is an ordinary application bundle.
The same way it has to, and it says so up front. Copy Shelter restores everything that was yours onto a macOS you installed from Apple, and the app walks you through that sequence before you ever need it.
Next: how to restore a Mac from a backup, start to finish.