You can but it looks bad and is distracting, a good blur doesn't distract from the rest of the photo and looking good enough people are more likely to actually

You can but it looks bad and is distracting, a good blur doesn't distract from the rest of the photo and looking good enough people are more likely to actually use it which is also important. You can also build a blur that discards enough information that it's not reversible and it /looks/ like they did that, Signal has been pretty thoughtful about security so far so I doubt they missed the research about simple blurs being insufficient to defeat facial recognition.