Why is the deadline given for refereeing manuscripts so much shorter than the typical time between submission and acceptance? On a few occasions, I was asked to review manuscripts (for well-known, mainstream journals) with a 2-3 month deadline.

I found this to be a short period given that the papers published in these journals showed about a one year difference between the dates when the papers were submitted and the dates when the papers were accepted. So I was wondering - why is there this discrepancy?

Do reviewers usually request for an extension of the deadline? Or is it common for reviewers to not take the deadline seriously (i.e. to ignore the deadline and submit the referee report after several months when it is done, and perhaps the editor needs to take the initiative to remind the reviewer to submit the report)?

I assume this one year difference is mainly due to the referee's delay rather than the authors having to make revisions (at least with my own papers, it often took several months before I received any referee reports).

While I have met the deadlines in the past, I wonder if I am perhaps allowing myself to be pushed around a bit in the sense that maybe I shouldn't drop my other work just to finish the review on time. I have reviewed only a few manuscripts so far, so I am new to this. I'm not sure how bad I would come across (relative to the norm, that is) if I don't take the deadlines seriously.
As an editor, I can say that it is very common for reviewers to submit reviews late, or to ignore invitations without responding so that it takes a while to give up on them and find suitable alternative reviewers. 

This doesn't mean that editors and authors don't notice and grumble about late reviews or notice and appreciate reviewers who submit their reviews on time or early! Please keep submitting your reviews on time if you can and ask for an extension rather than ignoring deadlines if you can't!

In my field, a 2-3 month deadline for reviews is longer than usual (my journal normally asks for a 21-day turn-around, and this seems typical of journals for which I am asked to review). But papers still frequently take a year from submission to publication due to delays in review, revisions, and final publication after acceptance.

However, I would also say that most papers are sent back for revisions if they are not rejected outright, and revisions typically also take months.