I really think it's because of the dining plan AND other incentives Disney gives to stay on site (like Magical Express) and not venture off for dinner. When I was growing up, most of our in-park dining was on site as CS places, then we would eat dinner back near our hotel off-site. But that was the 80's and early 90's. (We went many times back then.)
Then I remember when I went on a couple trips in high school (band and senior trips.) That was when you had to RUN to the restaurant you wanted to eat at that day when the park opened. I still remember my high school boyfriend and I running to the castle so we could get a reservation there that day. (that was in 1994.)
When DH and I went on our Disneymoon in 2001, there still was no dining plan or ME, so we had no trouble booking our meals the night before.
You can imagine my surprise when our next trip (in 2006) had not only ME, but a great dining plan. The
AAA agent warned us to make ADR's as soon as possible, which I found quite odd. Glad I listened to his advice, though.
So for our trip last month, I did everything as early as I could. Because of buying into DVC, I ended up coming across the DisBoards and learning all about the tricks of the trade (even though I considered myself an old pro when it came to visiting Disney.

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I have my trip for next June booked at our home resort (AKV) and already have the post-it note on my computer for when I need to call at 180 days for dining! (Right before Christmas!

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If you don't have a strong preference where you're eating, you should be able to get in to something (except during
free dining.) But the popular places book up so far out.
Good luck!