why do I find cheap trips on orbitz/skyscanner/ secret flyer but when I head directly to each airline's website I can't find that same flight?

For example, I found cheap (<$400) round trip flights from SFO to CDG in March. I have always used orbitz to find the cheapest flights but I always buy directly from the airline website. The airlines in question are Brussels and Lufthansa, operated under Air Canada and United, respectively. Does this mean that all the tickets are gone? I've never had orbitz or skyscanner not be up to date on prices so I'm confused.

Submitted October 14, 2019 at 10:16PM by not21forayear-_- https://ift.tt/2IPpD3G