Youtube is probably the one that you can't "Just Switch To Fediverse"
Youtube content is mainly by creators. If they won't leave, there will be no transition. And unlike reddit posts, you can't just reupload. Because they will copyright strike you and take it down. Also, videos take up a lot more space than just text and some low-res memes like reddit-type sites.
reddit is essentially a bunch of strangers talking to people, moving froms stranger Group A to stranger Group B is very easy to do. The reddit > Lemmy transition is probably one of the easiest. You're just joining a new group of strangers.
For everything else, your contacts will also need to switch.
For Mastodon, the people you follow will also need to switch. This is even harder than getting your friends to switch.