Because Pangram is snake oil.
If Pangram was snake oil (it's not but could be more mindfully marketed), it's definitely not now that every major lab has added watermarking.
You wouldn't even need to be able to read the information the watermark encodes to recognize it exists in a piece of text
2025 called and it wants it's take on AI detection back.
https://www.pangram.com/blog/third-party-pangram-evals
Note that while that page is from Pangram, the studies on it are not. If they are excluding studies that show Pangram performs poorly in realistic conditions, please share those studies!