Time Machine stopped backing up my Photos Library two years ago
Time machine constantly takes snapshots of each file, this simply is a lot of data, and being conservative is important for the sake of the consumer's end. Harddrive reads and writes are expensive! Especially when it is the eventual cause for a harddrive to fail and a critical system backup is therefore lost on the consumer's end.
Apple offers ~/Pictures/Photos Library to be backed up to iCloud by default, and is cutting a corner for saving your backup. Time Machine also doesn't backup many large system files that apple deems a &quot;defaulted&quot; file from macOS and easily reclaimed from remote apple servers.
Unrelated side-note: You can argue 5GB of free iCloud Storage isn't enough for even the Photos library alone, and you're right. But its incentive for you to pay for their service.