Ask HN: How come there aren't more “Kayak for x” websites?

Kayak, Priceline, Expedia, Trivago, etc. they all are selling the same airline tickets and are just aggregator websites, how come there aren't more for other industries? I go through the same process whenever I buy something online, I check the top 4 or 5 sites and Amazon for the best deal, basically what Kayak is doing but just manually on my own. How come the aggregator sites for travel work but you don't see them for other industries?