Why don't planets cause a solar or normal eclipse?

Answer 1

The inner planets, Mercury and Venus, are too small and too far away to block more than a tiny part of the Sun (less than 1%). So even when they are aligned with the Sun (which is veryrarely), they are practically invisible to the naked eye.

Transit of Venus 2012

Both Mercury and Venus make "transits" across the Sun, as seen from the Earth, at very long intervals. Mercury's transits are nearly invisible because of the distance from Earth, but happen about 13 or 14 times every 100 years. The next will occur in 2016. Venus is closer to the Earth and makes a shadow about 1/30th of the Sun's diameter. But because of the inclinations of the planets' orbits, transits of Venus only occur every 105 or 121 years, as pairs of transits 8 years apart. The transit following 2004 will be on June 6, 2012 and it will not occur again until the years 2117 and 2125. In 2012, the best visibility will be in the mid-Pacific and in Australia.

As viewed from an outer planet, eclipses by inner planets are very rare. The transit of Jupiter as seen from Saturn would be the most noticeable, about the same as Venus from Earth. But the transits can be more than 2400 Earth years apart.