I think the reason is that 'For' is required to combine with 'Siden' to indicate 'ago' as opposed to 'since'
Compare these English phrases. Many years ago (For mange år siden) and Many years since (Mange år siden). I hope my translation into Norwegian is correct. If it is you can see the phrases have a similar but not the same meaning. It is for this reason I think that 'For' is required.
I understand I should just accept that things mean what they mean but I'm so confused sometimes with the literal translations compared to what the sum of those words mean. For (too)..siden (the side) = ago. This sentence literally translated is "too many years the side". I guess it'll just take loads of practice to get this :P
In many languages, idiomatic expressions exist that can't be understood by analysing each component word individually. You just have to learn the whole thing as a unit - in this case that "for...siden" means "ago".
English has thousands of examples of this - if I give someone the cold shoulder, I have not actually given them anything, and certainly not a chilly body part. If I go on a wild goose chase, I'm not actually chasing an aquatic bird, wild or domestic.