Why not? It's because of the combination of distances, sizes, and densities of most of the celestial bodies involved. The gravitational forces that do cause the ocean tides and that do stress the earth's crust are almost entirely caused by the Moon, which is small and not very dense but is very, very close in astronomical terms. The Sun can add only a little to the Earth's gravitational equation -- it's big and dense, but it is so much farther away than the moon. Size and density determine the mass of an object and, therefore, the gravitational force it exerts. Distance determines how much of that gravitational force reaches the earth. It turns out, when you do the math, that all of the other bodies that will be lined up with the Earth on May 5 will be too far, too small, and too light to have any noticeable impact on the Earth.
Even if all of the planets in the Solar System (not just the May 5 participants) were ever to line up in a perfectly straight line on the same side of the sun that we are on, the total gravitational force that would be exerted on the Earth would be only about 2 percent of what the Moon already exerts. But is that enough to make a difference? No, because the elliptical orbit of the Moon already makes the Moon's gravitational effect on the Earth vary by 25 percent every month -- more than twelve times the maximum force that all the planets could exert -- and after 48 billion months (more or less) the Earth is still here. Besides, even though planetary conjunction on any grand scale are rare, they do occur, on average, about every 57 years.
So why are the nut cases saying bad things are going to happen? Well it turns out that they all want to sell you sensational books or survival gear or new religions or whatever. The media pile on because it sells newspapers or increases ratings or whatever. There's big money in disaster prediction.
It's not going to happen! But just in case it does, give $10 to Tom Wukitsch, and he will make sure you get priority seating on the space ship that's he has coming to rescue us. If the Earth gets wiped out and you don't get away on the space ship, he'll gladly refund your money. And if we do all escape, he'll give you back five when we reach the next inhabitable planet.
The "Bad Astronomy Harmonic Con(game)vergence" page: http://www.badastronomy.com/bad/misc/planets.html
(for other "Bad Astronomy " and "Mad Science" Pages: http://www.badastronomy.com/ )
Griffith Observatory: http://www.griffithobs.org/SkyAlignments.html
NASA Science News: http://science.nasa.gov/headlines/y2000/ast30mar_1m.htm
NASA Science for kids: http://www.thursdaysclassroom.com/
Waste $13!! Own your own example of the epitome of pseudo-scientific nonsense!!!! Buy and read this book quickly!!!!! Only a few days left before the end of the world!!!!!!!!
5/5/2000: Ice: The Ultimate Disaster http://www.amazon.com/exec/obidos/ASIN/0609800671/qid=954598042/sr=1-1/002-2981302-6886648
PS: The scientists don't have everything figured out either. The gravity of all of the visible matter in the known universe can explain less than 20 percent of the way the universe is structured. The rest is caused by unseen forces or "dark matter" -- a polite way to say that we don't know diddley about how it's all held together.