Not all professors customize their letters to the specific recipient. It probably helps a little (especially if the recipient is at a prestigious institution, so that they know that this letter isn't directed at a generic less-prestigious institution), but it's not that necessary.
But you definitely should. If you are applying to 100 different professors for the position of postdoc, and you send them a form letter saying "I am very interested in your work" without once mentioning what their work is, they are very likely to realize that it is a form letter, decide that you're probably not all that interested in what they're doing, and disregard your application.
So it's probably not enough to just put in their name, but you should also mention something about their field of research that interests you and explains why you would be the correct postdoc for them.
Is this a lot of work? Yes, but consider that you're competing for postdoc positions with a lot of other people who may not mind going to the effort of putting in a lot of work in order to get a job. Do you think professors want to hire lazy postdocs?