Because it is good for you.
Because you can
Because it avoids the slippery slope of skipping days.

Because it stimulates your bones and keeps them from getting porous.  
Karen

Because it works?

Why walking?  Because it requires no gym membership, no special equipment,
and is low impact so most anyone can do it.

Why everyday?  Because exercise needs to become a habit, like brushing your
teeth, in order to really work.  Doing something every day builds that
repetition so that it becomes just part of what you do, without thinking too
much about it.

Consider the lives of people in hunter-gatherer societies.  They walk
a lot
and they walk every day.  Our ancestors invented agriculture 10-20K
years
ago which is a blink on the time scale of evolution.  Our ancestors
were
hunter-gatherers from the time they emerged from the forests to the
time
they invented agriculture.  That's a span of roughly 5 million years.
One
of the general principles of evolution is that once a species has been
at
something for 5 million years they are tuned to do it by evolution.

It's an indirect answer to the question to be sure.  The principle of
evolution isn't a hard and fast rule just a broad trend.  There are
evolutionary pressures on humans like large brain size that resist
optimizing for walking.  Our hairlessness suggests we evolved for a
while towards becoming swimmers.  Humans suffer from knee problems
and so on.  But by and large, walking plenty every day tends to reduce
or prevent most of those problems not cause them.

There's also the anatomical set of answers.  The muscles involved in
walking are the largest muscles in the body.  Walking does count as
resistance work for the lower body and as aerobic work for the entire
body.  Walking can be done be all but a very small number and even
very large people can walk some and gradually build up their distance.

Walking is also social and humans are social critters.  Walking and
talking and socializing go together.  The more the neighbors are out
taking walks the more they know each other and the better care they
take of their neighborhood.

Walking is the one exercise that most of the population can do which
can be used to build an exercise volume base.

This Dr. Oz probably wants you to get an hour of exercise, which is
reasonable. But instead of recommending that you do a full hour of
aerobics or resistance training, which is probably quite intense for
most people, he makes half of that walking. This walking could be
divided into a warmup and cooldown walk before and after the main
exercise.

His recommendation seems to be quite compatible with the pyramid
concept in training. The training volume is divided into a pyramid
shape: a visualization device where the wide base represents light
intensity, the midle represents a solid training intensity, and the
small peak represents very high intensity.

Distributing intensity this way is important toward making progress in
exercise, balancing the need to stimulate the body into progress
against, the mental need for challenge and motivation, and the need to
recover and stay injury-free.

The every day aspect is interesting in its own right.  Most other
forms of
exercise come with suggested rest days.  Walking doesn't need rest
days
until you increase to the level of extensive hiking.

The very fact that walking is so healthy it doesn't need rest days
says
something.

After about 15 minutes of fast-walking, the tensions in my neck
disappear. That becomes a stress relief. When my neck hurts I become
aggressive. When my neck is free of tension, I feel good. To make
walking an even more pleasant activity, I bring my CD player and sing
along outloud. I used to walk in summer and fall. I stopped doing it
when the cold arrived. I've become pretty lazy in the last few months.

I like Determined's "no gym membership, no special equipment"
argument. :)