It all depends.

People get very attached for a number of reasons, some hate losing for others it's the sunk cost fallacy etc.. Depending on your system it can be difficult/impossible to revive them. Some tables keep characters, others use premade characters and/or swap them between campaigns. Generally you'd just roll up a new character and continue, sometimes at a slightly lower level or something.

Campaigns aren't necessarily linked together, most games rarely get that high at all. They just keep starting over the early game with different scenarios, for one reason or another.