Because they have no reason to do so.
Additionally, on Google you could be searching for confidential things you do not want intercepted, YouTube is more social and public, therefore it is not as high priority.
If you want to enforce HTTPS on sites, use an HTTPS everywhere plugin, https://chrome.google.com/webstore/detail/https-everywhere/g... https://addons.mozilla.org/en-US/firefox/addon/https-finder/...
Hence when google.com is forced over SSL when you're logged in, but not forced over SSL otherwise.