Agreed. I don't take vacations, so I wouldn't know about travel planning, but when I buy plane tickets (which isn't very often) I always use the same service. That used to be Kayak, but I changed it to Google Flights a year ago because their UI/X is way more simple and more intuitive than anything else out there (AFAIK).

So I too don't think the seasonal argument holds up, it's just about quality and marketing.