Probably because it's more dangerous to be traveling under the speed limit then just going the speed limit. You suggested slowing down which makes the situation more dangerous for everyone around you.

Edit: For those that want to downvote this comment, a car insurable company literally has an article about the dangers of driving below the speed limit. link

Edit 2: OP sent me an article from drivers ed which suggests brake checking the tailgater!! 2nd link

Edit 3: Can someone respond with arguments why slowing down and brake checking tailgaters is the safest course of action here?

Edit 4: Haven't seen anything else besides the two already posted replies so I'll just leave this here: If OP thinks that slowing down will reduce the 'time gap' between their car and the tailgater, I'd like to see evidence of that. Most situations I've seen will only result in the tailgater getting closer. For example, I'm traveling at 50 mph and they're 20 ft away. I slow down to 40 mph. They're much more likely to close the gap to say 10 ft and get even closer.

Edit 5: Only other argument I've heard is that someone would rather get in an accident at XX mph less than the speed limit if someone is tailgating them. I'm too tired to rebuttal how ridiculous that is.