You’re asking a good question, but I don’t have the answer. I don’t usually use Kayak.
There’s more than just safety reasons to avoid specific model of plane. While both are Boeing, a First Class seat on a United 737 (of any variety) is a subpar experience compared to a First Class seat on a United 787. If you’re making a long trip and paying the top dollar for that, filtering out the 737s and A320 planes makes a lot of sense.